I'd atl least try to run a bucket of Equisure through him. My guy, back when he could chew hay, got water with manure balls that lasted for months with no improvements. Equisure for hind gut acidosis and ulcers cleared it right up and it hasn't been back. What kind of hay are you feeding? Sugar levels can promote acidosis in the hind gut. How old is the horse? I'd consider hind gut acidosis before colitis, though colitis is a possibility, just much less common.