How does he respond when asking him to walk backwards when you are not riding him?

Are you just using the reins to stop / slow down?
Horses are into pressure animals & if he's in pain or been caused pain in his mouth before, he is likely to pull on into it.
Remember, it takes two to pull!
Try just sitting your bottom down in the saddle when you want to slow without touching the reins, if he reacts in the slightest to this aid - praise, praise, praise. Likewise, when moving off from a standstill. just lighten your seat a little to make it easier for him to use his back & then sit in balance.

What bit are you using?

Physical things that need to be checked are: Teeth, Back, Saddle Fit, Bit & Bridle Fit, these are all very important to sort out as if he's in pain, he is unable to respond as you would like him too.

to answer vicci
he seems ok without me on his back doing backward transition however again a little bit of lowering his head

and for beryl
i am using pessoa bit. and for the exercise i tried a cowboy exercise, canter halt canter, and without rein he was also trying to lower his neck and putting his weight on his hands so i stopped him sharply with using reins and then he was stopping using more of his hind legs and i praised him. may be i should continue this for a while!!

sorry he stopped lowering neck and used more of his hind legs

the other serious problem that i have with him is rushing while i am asking flying changes ?

something to mention :
he also bring the head down when i am asking for a turn in long lining , not sitting on hind legs at all?
what should i do ?
i also make a new post for this :)

it sounds like you may be asking a bit too much of him before his muscles are strong enough to cope with these exercises especially as you say he's on his forehand on the longlines.
Rushing when asked to do fling changes is also a sign that he's a little worried and not quite sure what you want.
Canter, halt, canter is also an exercise that needs strong muscles. Can you not give him more of a chance with progressive transitions? IE, walk - trot, trot - halt - trot, trot - canter, canter - walk - halt.
personally, i'd go back a bit in his training to more steady, strength building work for a while until he is more able to cope.
Hacking out / Trail riding over all sorts of different ground is an excellent way for them to use their bodies as they should do & get fitter.

I agree with Beryl, seems like 'too much too soon'. Also, you mention its a Pessoa bit, but that's the brand name, what style is it? Is it snaffle, does it have long shanks, or some other style? Bits and reins are for gentle communication only they are not leverage for stopping - the force on, for example, a bit with a long shank is massive and can cause extreme pain and misery for a horse. Do you long-line in a bit? It may be an idea to try everything in a Dually halter (including ridden work) and see what the response is then.

Absolutely, try riding in the Dually, the mouth is sacrosanct!!
There is only a very thin covering of skin over the jaw bone & nerves, if you use the reins "sharply" as you've described, you will cause damage, he's trying to tell you by leaning into the bit - Horses are INTO PRESSURE animals, they push into pain!
You must use your seat & body to ask for transitions, your hands on the reins are ONLY THE LAST LITTLE BIT of the aid & you need to work towards using your seat & body aids only.

well thanks every one
i try the dually and let you know