Why does my kitten poop on himself so much?

We have an 8-week-old kitten that we recently got from a shelter. He (thankfully) uses the litter box, but he frequently leaves poop on his rear end end which then gets on his hind legs, tail, and other places when he curls up and sits down. He had been having problems with diarrhea, but the vet gave him some meds and a dietary supplement to fix that problem. Now his stools are more formed and less frequent, but he still gets it on himself pretty regularly. Is this just a common problem with little kittens? Or could it be some other problem?