Try bending outside and in! My horse was very flat and after a few months I can do anything with her head and she is still straight!
have the dentist come out and check teeth and a chiropractor come out and check him out
two of my warmbloods did the same thing...one did exactly what you described, got his teeth floated, and he was as good as new. my other, however, in addition to what you explained, exhibited mischevious behaviours, and it turned out that he had a jaw condition.
Could be both. When was the last time he his teeth done? A horses teeth should be done every 6 months so if it has been over that, them you should defiantly get them done.
My horse had a similar problem, the chiropractor came out and there were a few problems but I also had to address a lot of muscle stiffness going on.
One of my trainer's horses had a similar issue, she had the chiropractic out and it turned out that there were several problems. So calling a chiropractor is what I would recommend.