Constipation was going to be one suggestion, but also is it solid or runny?? Or is he possibly straining so much its hurting to come out (which would rule in with constipation, I suppose). Another thing it could be, if he had diarrhea, was that he could have 'ripped' or 'torn' his anus, causing it to be painful when he went?? I don't know, I'd keep an eye on him and definitely bring him to the vet if it persists.