I had the same issue with my horse for the longest time. We found out it was the timothy/alfalfa we were feeding. T/A tends to be the go-to hay around here, but it was too coarse for his sensitive stomach and it’d make his manure really loose. We’ve switched him to orchard/alfalfa and he does great on it, as orchard is a bit of a finer grass than timothy. You might want to try him on a different hay if you can, to see if that helps.