If you post a handful of shots that are fairly representative of his work, it might yield better tips.
Off the top of my head:
- Shooting from the side or slightly below is more interesting.
- Get as close as you can to your subject to minimize noise and add color.
- Use an external flash, possibly a diffuser and learn how to aim it properly.
Know the limits of your setup. Wide angle is very difficult to get a great shot, because you typically need some distance between you and the subject. If you don't have the strobe capacity, resist that great WA shot, it will just dissapoint. Instead focus on what your camera does well, you will get more shots that way.
I don't know how good a diver he is, but somone who struggles with bouyancy should not dive with a camera for safety reasons (their's and their subjects), also they will not take as nice a pic if they are moving in the water column. So if this describes your 15 YO, I suggest taking a few dives and getting his bouyancy and body under control, then go back to the camera. You really can't learn both well at the same time.
Composition is subjective to the viewer, but is something that always needs work. Learn the way your camera sees things, and work with in those limits. To many people (myself included) forget the way their camera looks at teh world and are then dissapointed in what was captured vs what I actually was looking at. WA especially.
Learn post processing. I hate it, I stink at it, and I think some of the shots I never let anyone see would be OK if I were any good at it.
Other than that, throw some of his better and worse pics up here so we can see, and maybe have some more ideas for you...
edited in:
Point and shoots do not always do contrast well. As in a dark fish on a white sandy bottom will either be over or under exposed. Try a few but don't get frustrated if they don't turn out well.