He is using your body for the advancement of his education.
Once he gets a license, he can charge you if he chooses.
If you really want to do something nice, offer to babysit their kids if they have them so they can have a night out. Get them a gift card at a nice place where they can have dessert and coffee. Something like that.
When my daughter was going through beauty school, she did lots of people's hair on private time and she wouldn't take any money. She considered it practice. She did, however, accept jars of homemade jelly, baked goods, Starbucks cards. Oh...and jars of pickled beets. Her favorite.
Don't get hung up on the "tipping" idea.
He asked to be able to practice on you.
Doing something nice in return is great, but since you are such close friends, I wouldn't consider it "tipping".
Just do something nice that you know he'd never ask for.
I'm sure he will appreciate it.
Best wishes.