Vet. Diarrhea can cause dehydration very quickly. You can do a tent test and that's gently pulling up on the skin on the back of his neck and if it goes right back down, he probably isn't dehydrated. If it goes back down slowly, he probably is. Do you have any unflavored Pedialyte you can give him? Is it smelly or a different color than normal? If so, then I'd say get to the vet even faster. That could be a more serious illness. Is he eating and drinking?
Also just wanted to add that he may have just eaten something that upset his tummy and I wouldn't normally suggest just running to the vet, but if he's deydrated, not eating or drinking, it's discolored or smelly...he should go. The red flag for me was you saying he was acting differently as well.