You may want to start out as simple as possible and let him move up if he gets into it. It takes good buoyancy control before you start taking pictures. As mentioned above, the Canon's with their own housing are a good way to start. I have invested mega bucks into camera equipment over the years and it gets obsolete fast. Resale on these items are not worth the effort and I would rather keep my stuff as museum pieces than give it away.
My other suggestion is for you two to attend some underwater photo clubs. There are a lot of Underwater Photographic Societies all around the world. Most of the members are very willing to share their knowledge. Had I not joined NCUPS I would have given up in frustration after a short period of time.
Good luck.

Harriette
Take only pictures leave only bubbles