Are you sure there is nothing wrong with his health?
Sometimes things like these happen when there is something wrong with their back or teeth. I knew a horse.. she always wanted to work, she was never grumpy and all of a sudden she stopped working. She didn't want to do a single thing and all she did was just stand there. She wouldn't move, until a vet came to see her and discovered it was her back that had caused all the problems. They treated it and a couple of weeks later she was like before her problems with her back!
I am not saying there is something wrong with your horse, but keep in mind that things like this can happen.
Edit: I see your horse has passed a check and everything was fine.
What do you think he’ll do when you give him a break? Don’t do anything with him for a couple of weeks and see what happens then? Sometimes that works out great!
A friend’s horse was acting the same like yours and she tried everything just like you (vet check, UTD, teeth, saddle) and she did not know what to do.. someone told her to give him a break for a couple of weeks. At first she thought ‘’why should I do that?’’ But she did, and it worked out just fine! I think he needed it.
When she started training again he was like nothing ever happened. She didn’t even recognise her own horse! And when her horse gets grumpy again she gives him a week off and everything is fine. Examples like these show you that there’s not always a reason for behavior like this. I’d say to you, give him a couple of weeks off and see what happens!