Could be worms, anal gland problems, allergic itching, etc. or simply something stuck to the bum area. My kitty scooted when he had diarrhea due to a diet change- the soft poo stuck to him and he was scooting to get it off, a.k.a. "using the floor as toilet paper."
May need to have his anal glands expressed. The vet or a groomer can usually do this at a minimal fee and show you how to do it (although I understand it tends to be the type of thing you'll gladly give the vet a few dollars to do!)