Check his teeth and the inside of his mouth, he may be in pain. I’d definitely get that checked out by a vet before doing anything else. If he comes back fine, perhaps changing his bit.
You can put out your hand like you have a treat then he will put his head down to see if there’s food
tickle his tongue 👅 it works everytime!
He may need his teeth floated. Some horses do that when their mouths are sore from sharp teeth.