Three questions I always ask, in the order I ask them.
1 Am I certain The horse is not in pain or physical discomfort?
2 Am I certain that the horse is capable of what I am asking him, is it a natural progression from what he has shown he can already do?
3 Am I certain that the horse understands what I am asking?
I would say it's a no to all of these and there's you're problem.
Firstly just like if we suddenly went to the gym every other day, we'd be sore or at last feel weaker in some muscles and stiffer in others.
Secondly he is very likely not capable of what you are asking him at present.
Thirdly like most horses 'pulled into a shape' he has no concept of why or what the purpose is and therefore it feels like torture.
This is not aimed at anyone here or the OP but being from the UK I actually moved from my last yard because it was all dressage. High level dressage at that. I couldn't see it everyday, too many horses being tortured day in and day out. It's not beautiful it's horrible. Sorry rant over! My point is that dressage movements should be q thing of beauty. They should be the horse at his most powerful and graceful. Instead modern riding is often like a loaf of White bread. It looks like a loaf but really it has no substance. ( I think it was Chris Irwin who used that analogy)
Show your horse how to move in balance and collection on the ground first. That way the horse will become more flexible and powerful, and more importntly feel the difference himself. Then he will understand what is being asked from his back, without pain or soreness, and more importantly he will have a reason to do it.
Too often trainers squeeze the horse between the legs onto the bit contact, and it's like driving with the accelerator and brake on at the same time. Even at a high level 9 out of 10 dressage horses are NOT collected. Don't believe me? Then a simple test. Release the bit pressure and see if the horse holds it. He won't, he can't because he has never learned to balance and hold himself up. The reins become a crutch, and the horse is still predominantly weighted on the forehand.
So in summary - try some passive work with the horse. Stomach lifts, croup and shoulder presses. Get HIM to stretch and hold it, (ie using a carrot or whatever if necessary) Then try lunging him in a restricted area. Somewhere that is smaller than the diameter of your line, and try to get him so that he is not leaning on the line, so there is a considerable slack in the line. The line should be used to communicate subtle signals, ideally to reinforce those or your own body, not to keep the horse on a circle. Then he has a chance to find that lower elongated position firstly, then from there bringing in the hindquarters underneath, where his neck will begin to shape into ramener too.
Work with the horse and he will most likely work with you if what you do makes him feel stronger, more confident and powerful.