I agree that he should be seen by a vet asap. Kittens can have worms, coccidia which causes diarrhea, fleas, and other medical problems. He should also be tested for FELV/FIV and if he's healthy he should get his first shots.

Later on he also needs to be neutered so that he's won't start spraying and marking his territory, getting into cat fights, wandering away from home, or getting female cats pregnant. I sure hope that there's nothing physically wrong with him. Good luck and please continue to keep us updated about him.