I would recommend doing a bounce or gymnastics to make him pick his head up after a jump, if he crashes it’s actually good so he won’t do it again and he’ll learn not to put his head down.
First of all you need to see with your vet if he doesn't have back problems.
How old is he? He's used to jumping? Usually, young horses do that to have fun, they stop with work and age.
It comes from a lack of balance. You can try to stop him right after the jump, try to put a ground pole after the jump, it will force him to find his balance and be focused.
It can also comes from your position, if you stay too much on his shoulder at the landing or don't go with the jump with your hands, he will try to escape by running away.
You can make mechanization lignes so that he stops running away.
I hope I could help you, good luck :)