Kids and Plyometrics

Look online for the PEP program.  It was designed for girl soccer players and greatly reduced knee injuries.  I used a reduced form of it for 5th and 6th grade co-ed team, and I it worked for us.  It has some plyometric exercises in it. 

Teach good technique and body control, and then let him hit puberty.  Plyometrics done with poor technique (quad dominant jumping/landing, which is seen more often in girls than boys) put one at risk for injury.  Over the next 4 or 5 years he'll be getting an increase in testosterone which will help increase overall strength anyway, and his vertical leap will improve on its own.  Plyos can be introduced during or after that time, IMO. 

If you're looking for research, check for the NSCA's position statements on plyometrics and explosive exercise, or for programs done by Tim Hewett, who's done a lot of research on knee injuries and training programs in basketball/volleyball players.  I can look around for them later but I don't have the links right now.