Use your best judgment on when you think he's ready. Make it a point for you two to shoot together as much as possible so you can help him out. He could be ready in a week, or two months, but remember he'll never truly learn the sheer challenge without experience in the stand. Heck, he may not even get to full draw on a deer, or he may shoot a perfect shot...hard to predict, but you know how experienced he is in general when it comes to hunting.
I believe the best experience is in the woods. Also, IMO, the chosen stands are up to the hunter with the ownership/rights to the land and the guest gets what is left.