does he get grain? Some horses will pick through their poop to get to undigested grain.
If he’s stalled all day it could be from boredom. Maybe put a hanging toy in his stall.
And as the others said, maybe try a free feed with him so he has hay all day.
If he’s stalled all the time, is there anyway to make him a larger turn out?
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."