It is OK to let him have the controls but don't try to be his instructor.
I'm surprised he wants S&L practice if he has already done his first solo x-country trip. I suggest he would get far more out of the flight by sitting in the RHS without hands on and mentally flying the trip. He could judge how far ahead of or behind you he is in what he would be doing and when and would probably get more out of it than concentrating on handling the aircraft.