Hitting, punching, scratching & pinching.
Does mom know this is happening? What does she say?
You could try Nan's "bye bye outside" approach and see if it changes. Essentially, they need to do their goodbyes before opening the door.
But honestly, it sounds to me like he may need a different environment. Nobody should have to put up with violent behavior. Do the other kids you care for see this? Does it spill over to them, meaning does he treat other kids this way or just you?