Could it just be that your dog needs wiping after he goes to the toilet? I wipe my dog's bottom occasionally when we get back from a walk to avoid this exact problem. He doesn't have any health concerns either, but sometimes if his stool is a little less firm than usual, I feel like it's something you need to do.
Otherwise I'd go see the vet again and get more advice. Maybe he's due to have his anal glands expressed?
Good luck!
