Having already blown out an ACL, MCL, and did meniscus damage all by 20 years old, here is my advice. FIrst of all, harry is correct in saying that knee braces aren't always the best idea as a preventive measure on a "tweaked" knee. There is not a knee brace in the world as strong as a healthy knee, as I am told over and over by my orthopedic surgeon. And more importantly, if knee braces worked as a preventative measure, why do all wakeboarders blow out their knees??
I would greatly suggest that you never use the velcro on braces with the little metal hinges type thing, they are crap. He would be better off healing the knee up and doing some serious rehab. The best prevention is building strong muscles around the knee, especially quad, hamstring, etc.
I realize however, that your situation calls for some help for his camp that is coming up soon. The only brace that will do anything positve is a custom brace, such as the CTi2, which I highly, highly reccommend. Others swear by the DonJoy brace. They are expensive, but worth ever penny when you land legs locked on a hs front to fakie. They are the best, and unfortunately, they are the only option. Buywake and others sell some 200-300 dollar braces that are slightly better then the typical 30 bucks braces, but if it were me, I'd go for a CTi2. Blowing out a knee hurts far too much to risk it, and 8-12 months on the couch is no fun either. If you have a good orthopedic surgeon, explain the sport thoroughly to him, as well as the situation, and tell him the CTi2 is the way to go for wakeboarders. If he'll write a prescription for it, you may be able to get insurance to cover it. Blue Cross covered all $1500 or so of mine, thank god.
Good luck and stay healthy!!