Is he lethargic or is he still acting like his normal self?
Have you noticed him eating anthing he shouldn't (e.g., tearing up anything such as toys or bedding)?
What have you been feeding him lately?
Has he lost his appetite or is he still eating?
Is he trying to poop but nothing comes out?
Anyway, watch him close. If he seems lethargic, not wanting to eat or trying to throw up but nothing comes up, take him immediately to the emergency vet - one or more of these symptoms could indicate a bowel obstruction which is life threatening.