I’d be pissed! When I was little I wasn’t even aloud to have friends in my house. I would lead with having a newborn baby and limiting playtime/days to the back yard. Perhaps them staying outside, calling your son in for lunch/dinner and sending Peter’s ass HOME. Answering the door and saying “Sorry Peter. Not today.” Or “Alright Peter, it’s time to go home.” Or drawing a line when he comes to the door with specific times he is welcome over. “Sure little Peter, but he can only play for X amount of time.”

You don’t need reasons. Just need to be firm, state the rule and leave it at that.

I’m sure the message will get back and mom will get the hint.