If he has plenty of feed, not under stress, doesn't have worms he may have coccidiosus. Cows with this disease are usually described as poor doers. Take a fecal sample to vet to check. If thats not an option and you have access to a microscope, smear some fecal on a slide and look for little round egg looking masses. You can get pictures of them on the internet. Fairly easy to treat but is contagious. Treatment is with Corid - I think.