I agree, it certainly sounds like he has worms. Most all puppies are born with them and have to go through two dewormings in order to get rid of them completely. See you vet and have his stools checked; once he is wormed make sure and keep your yard clean or the problem could reappear again and again.
My first question would be, has he been wormed and, if so, how many times and with what? Do you see any round worms or tape segments in his stools?
If he seems fine, I think I would take him and/or a fresh stool sample into your Vet nest week.
At this point, this is only a guess and definitely keep an eye on him.