May be pasture is too rich? And you might want to have it tested to see by USDA agent, which is usually in bottom of courthouse here, but easily found.
I'd be willing to bet the difference in grazing is a lot of the problem here. And conversely, while you say no teeth problems, you haven't checked either, he could have bitten down on rock, cracking a tooth, or have twig, thorn or piece of wood stuck in mouth/gums/tooth and be more hypersensitive to stimuli because of that pain.
Horses make me a better person.