^^^ ditto too rich of a diet as a possibility.
Also, if he has a lot of leg hair, has anybody checked for Scratches (a/k/a Mud Fever) under all that leg and fetlock hair?
That stuff itches/pains like nobody's business. Some horses tolerate it better than others.
It's most likely too cold for ticks or mites, if you folks have anything like that
Tick bites can really make a horse crazy.