Vaco... Here is the main issue. If the boat you are teaching in is not comfortable then you risk him not getting into it due to lack of comfort. Then you push that further with a boat that probably doesn't fit and you have lack of performance issues. I would at least consider renting a Fun 1.5 or 2 fun depending on his size.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body. But rather...To skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming... WOW !!!! What a ride!!!!!!"