Hi Jess, I cannot help you here as my knowledge of saddles is so limited however. I wonder though whether it would be worth reversing the process and exploring how you can 'even him out' with the help of your physio/osteo team before you begin to think about a saddle. Many horses are lopsided because we do everything on the left side including mounting so the muscles get over developed on that side (just like us). Also, if you get one now and build him up his shape will change again and so the saddle will be 'off' again....just a thougnt :-)

Hi Jess. Since I haven't seen your horse or your saddle, I can only offer some thoughts and hope they can be of use to you.
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Generally speaking; type of saddle is very individual and sticking religiously to one or the other will become counterproductive in the long run. You always have to regard the horse and rider together and all circumstances.
A treeless saddle has many advantages as it shapes itself after the horses back when it changes and does free up the shoulders (depending on make and model of course!) BUT it does not distribute the weight very well. There is no tree for the stirrup leathers, so the weight of the rider whenever more weight is put on the feet, will be centered around a very small area. It also demands a well balanced rider since there is less "help" from the saddle itself.
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A saddle with a tree should - in my opinion - be one with an interchangeable one and fairly easy to "mold" after the horse's shape. It also needs a very good saddle-maker to help with the fitting and checking up on it annually.
On the flip side, they are very good at distributing the riders weight, aid in keeping the balance and - if well fitted - will slide around less than most other saddles. There are also any number of make and models to choose from.
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The saddle best suited to distribute a riders weight, would probably be a Western (or possibly a baroque) saddle. Of course, they are in as much need to be fitted correctly as any other type - the treeless ones included!
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The weight of the Western saddle vs a treeless is of much more concern to us humans who have to haul them up on a 16 hh horse ;) than it is to a horse that weighs a hundred times more than a saddle.
If the horse has a problem with the *weight* of any modern saddle, it should be considered having a vet out to check him up.
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As for your current saddle I believe that either you and your horse have twisted it. Or, it was twisted to begun with and it took some time for your horse to get uncomfortable from it.
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I would suggest that you start by taking your current saddle and horse to a good fitter of saddles and let them have a look.
It might save you both time, effort and money in the end if they can fix it.
At any rate they should be able to suggest what kind of saddle that will fit and also fix it to mold.

Hello Jess, Vicci and Kicki; I, like Vicci, am no expert in saddle wizardry. Kicki, thank you for sharing all of that knowledge you have on the different saddles-the why`s and what-fors. That is really valuable information. I do know that saddles change themselves over time and become mis-shapened. I also know some saddles are very expensive and not everyone has the capability financially to go out and buy just any saddle they choose.
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.With all of that said, my thoughts go back to the third paragraph that Jess wrote about her Woods being mis-treated and having back problems for along period of time with the possility of Woods holding [storing tension] in his back area. With all due respect, I don`t believe that horses store tension in their back. Tension is usually stored in the neck area and is transported to another area of the body because of injury and or unnatural movements cause by external stimulus [ill fitting saddles]. My suggestion to help your guy would be to soften-up and stretch the neck while messaging the crest area; find out where the soreness in the back is located and message in circles ["T" TOUCH"]; and when the initial soreness is gone in his back, ride him for conditioning beyond where he has ever gone before in just the walk [collected and swinging] for the rest of the year. This will build up the back muscles while creating more length of stride and hopefully bring about a healed sore area in his back..
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Jess, you have spent 7 yrs with trying to help Woods overcome his problems with this issue. He knows this, but the nagging, ass pain keeps cropping up in his life. Give these neck stretches a try and get his nose on the ground for his relaxation. He will tell you where the problem area is in his back. Then it would up to you to unbundle the twisted nerves in that area. Then look into Kicki`s suggestion as to the saddles. This is going to take some time to bring about, but it`s such GOOD WORK.
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I can point you toward some videos that you watch for reference of that collected swinging walk if that sound s good.
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Please keep us posted

All the best
Bud

P S. The videos that I mention above can be found on U-TUBE under VIDEO CRITIQUE: LISETTE AND TIMO SUBMISSION #2. They originate in Holland, I believe, and you get to watch a lady take a 15 yr old horse on a journey toward ultimate conditioning under the watchful eye of Will Favour, a dressage instructor that I deeply respect. It will give you a first hand view of how to condition a horse for change in how they carry themselves which would possibly eliminate Woods`s problems. The #1 session of the video would be helpful to start-off with to get things going. The magic is in the neck and how we get him to use it. Good luck and please let me know what you think.
Bud

Wow thanks guys! Lots of really helpful thoughts/info.
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Vicki I hadn't even thought about that so will hold off then until I have built him up a bit. I have to say that when I look, watch and feel his back muscles do look even but maybe I'm missing something subtle. Although contradicting that I feel he is less balanced on the left as a whole.
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Kikki I am reading about balance training so am starting to work on that. I have also been reading about different exercises to strengthen his back, seeking the bit (though I do ride in the dually) and all that kind of stuff so am hoping this will start to help.We always do our carrot stretches in the morning (don't tell Monty-woods wont do it for free!lol) and he seems really flexible. He used to be very tight in his shoulders as yes he held tension there. He was and can still sometimes be such a tense horse and you can see his neck, back muscles tense up big time when this is the case. (has always been spooky but when he spooked before I had him people used to beat him so he always expected pain when he was already scared and then spook even more. By the time I met him he was terrified of every sound, movement, person ect-so a very tense horse/ very tense muscles.)
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For such a long time it's been just to get him to relax, listen and have fun and turn all this around. Feel like at 22 we are now starting to actually work on the athletic issues instead of mainly the mental.
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And thanks for the saddle advice. I have been toying with the idea of a western saddle but woods loves jumping so still want to do that with him but it would make for a more secure seat doing the archery! If only I had money to have both! Will have to keep reading and thinking about that.
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And Bud thanks I will check all those videos out and get started. there are time I wish i could just get on, do whatever and not have to constantly re-think everything. Impatience!! BUT that is Woody's gift, he challenges me all the time, makes me think from a new angle, push myself in my learning and riding and if it helps him I don't care how long it takes. Challenge accepted!
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Thanks again for your wisdom guys, will do my homework, get going and keep you posted! Jess