Get a chiropractor to check all of his body especially back
If it isn't pain related, I would try a running martingale
My horse used to do this, especially coming into Jumps. Maybe try just lightly squeezing the reins before a jump. I used to count to three while doing this coming into a fence and it really helped, now my horse doesn't do it at all.
I have a friend that had a horse who did this. It ended up being allergies. They gave him mess and supplements. So it might be allergies
All of the aforementioned could and still can be factors. There are a few horses I know who would toss their heads when their tongue was above the bit or simply out of boredom. To combat this, a bit with rollers was used and the later horse started doing cavaletti training. Hope that helps!
Hmm.. I haven't actually should probably get on that. Thanks 😊
Have you checked the saddle fit and the back muscles. My horse won't flex over her back if her back is sore, and a lesson pony I know had a saddle that pinched his withers. So every time someone leaned forward, he threw his head up.