2 year old and my teenage step-son

I have a 15 year old step-son (Josh) and a 2 year old grandson (Chance). Honestly, I do not like the way Josh plays with Chance. Josh spends more time trying to discipline the baby more than playing with him. Through the whole play time, Josh is like, “Chance don’t touch this don’t put that in your mouth, you’re being a bad baby” and the list goes on. I would prefer that Josh does not play with him period. Josh thinks he knows everything and I need to be able to talk to him, and explain to him how to play properly.

The end result of their playing is Chance hauls of and slaps Josh in face. Help!