that dog has worms!!!!!!!!!!!
It can also be from worms.
My pug used to do this constantly. Her anal glands would get infected frequently and needed to be emptied by the vet (I could have done it myself but it was quite disgusting) It happened so often that we were bringing her to the vet every month, and even that wasnt enough. We finally decided to have them surgically removed which was worth it in the end because she no longer scoots and isnt uncomfortable constantly (and doesnt have to go through the stress of going to the vet every month) I'm not sure if that's what's wrong with your dog, but it wouldnt hurt to bring him to the vet to have it checked out. The scooting could also be a sign of worms. Either way, I think a checkup is probably the best thing so you can find out for sure whats wrong and what will help.