I would get a fecal test done for parasites. It could be pin worms. I've been told you can check at night with a flashlight under his tail. The vet will recommend the correct wormer, if that is a problem. Sometimes you have to eliminate possibilities, in the search to find the real reason. The vet has done a skin scrap on mine in the past to check for lice/mites.
He's a cutie! I like his little dark nose.