Having certified more than 50 kids in the 12 to 15 y/o range over a period of more than 5 years, here are some thoughts:
First, and most importantly, there is NO way you can control a kid just wearing JUST a wetsuit, regardless of the buoyancy issue. To hold onto him underwater and exert SAFE control, you need STRAPS. That means a BCD. If the kid panicked and shot for the surface, he'd be gone before you could stop him. Second, mixing free diving and SCUBA is risky at best because even adults have problems remembering when or if they are supposed to exhale, so I hope this wasn't the case here...
I had a 13 y/o panic during a buddy breathing drill at 30 FT and shoot for the surface obviously holding his breath. In this type scenario, the first priority is to STOP and regain control of the ascent followed by forced exhalation. I had one hand on the kid's straps, the other hand dumping my BCD and the kid's BCD air and my knee in the kid's gut forcing him to exhale. We successfully made the surface without injury, but those types of scenarios are never easy.
Third, YES, allowing kids to experience underwater breathing is a great idea, but I'd always start in standing depth water or less, never went deeper than 8 FT and always did it in a pool. Again, CONTROL is key, so you better have that figured out before you attempt it.
Finally, yep, I'd talk to the parent afterward also, not so much to "bust" him, but certainly to inform him of the risks. Doing it in a "positive" fashion is the way to go here...
So, ScubaFool, I got your back brother....