I bought my 10year old daughter the transpac from dive rite. I dive one with a rec wing, they have sizes down to XS but I bought the small as I can size it down for now, but will last her a long time. It will easily adjust for any size tank as she will be using al40's to start. I got the trek wing which is very small and compact and gives enough lift for up to a steel lp 100. As she gets older, I can get her mediaum shoulder straps to extend the life, and when the time comes to upgrade her harness, the wing will fit the new harness.
I dive a transpac because of the freedom of motion and the light weight. Check out dive rite express at
Dive Rite Express, they have a few videos so you can see how it works. I bought hers from dolphin scuba via ebay for exactly $500 for transpac harness, travel wing, crotch strap and 16# ditchable weight pocket. They shipped it fully assembled and it was here fast. I did a "make an offer" on ebay.
If he doesn't use it much, they hold value pretty well, expensive rig for a kid, but the light weight and ease of use will make thier diving more enjoyable.
As for the tanks, most LDS will rent AL63's. Her sac rate is so much lower than mine that she is fine with a 40 if I use an 80, if I use a 100, then we rent a 63 for her. (the 40 also doubles as a stage bottle)
Diving is a life long sport. Kudos for getting your son in at such an early age.