Some basic advice,please

Forgive my ignorance. I'm totally new to TeamViewer and I need to know how best to use the software to help with the following:

My Granson is appraoching some examinations and he's weak in a subject. His school teacher has given him some Computer based software for his revisions/strenghtening and I want to help him through this work.

I'd like to watch what he's doing on his screen while conducting a video link in another window.

He has a MAC m/c and I use Windows. We've both installed TeamViewer and are about ready to go but I'd like to use the best configuration. Should I give him control on my m/c or should I take control on his ?

Or is control not an issue ?

All advice very much appreciated.

Brent