He is more than likely expressing his anal glands...if there poop isn't hard enough to express it when they poop...than they rub there butts to express the rest.
In your other thread you said you were having problems with your pup wolfing down his food, thought you weren't giving him enough food
New here and puppy questions
Did your vet give him his shots when you took him in? if so that could be why he isn't eating as much right now...the shots make them sleepy and sometimes up sets there tummies. And did he have worms for your vet to give him worm medicine for? That also could have been why he was wolfing his food down if he has worms and rubbing his butt, but if he didn't have worms I'm wondering why he would give a 8 week old pup worm medicine