I agree with a chiro, and also an equine dentist. He's at the right age for wolf teeth, so I'd have those removed if he has them still. I'd also have his inner ear checked, as the poll of the bridle could be pressing painfully on something up there.
Also, keep in mind that while he is 8yrs old, he's still very green. He's likely not well balanced or well developed muscle-wise (no matter what he might look like). Once you get the head tossing thing figured out and rectified, I would work consistently on getting him back in shape and better balanced, using trot poles and caveletties, and lots of trail riding if you have trail access. We often forget that even mature horses can be unbalanced and weak (in the muscle groups needed to be ridden) due to inactivity or lack of proper riding/training.
"To be an equestrian in the classical sense is not just to be a rider. It is a position in life." --Charles de Kunffy
Anti-Slaughter and Proud!