Have you changed his feed, added more feed to his ration, has the weather taken a turn? if all the answers to these is no, then it could be a medical condition. Cociliosis, (not sure on the spelling) or something like it. My buddy bad his steers come down with something that came from the water trough, it got there from the crows that were bathing in the trough. He ended up medicating and then drenching with butter milk, the butter milk really helped get the guts going right again, pro-bios did not help for some reason.
If you think it could be medical, call your vet and see what he recommends.