By the way, what kind of program is he doing? At that age and through the next few years, it's probably the best time to introduce compound barbell movements and you can gain a lot of strength quickly (as long as you're eating enough). I know people caution about injuries, but as long as you are using proper form, I think he will be fine. My son is 13 and is going to start going to the gym with me soon (he's been after me for awhile), and I told him he had to read Starting Strength first to get an idea on what he's doing. He's just mostly done body weight stuff regularly so far.
I first started lifting weights regularly when I was 13. I was probably around 150 - 160 at 6'01". By the time I was 16, I was around 200 lbs and squatting and deadlifting in the 500's. It's all about learning and using good technique. Yes, every kid is different and develops at a different rate. Just go slow initially and learn good technique.
The other thing I would say is make a concerted effort to focus on flexibility / mobility. I did not and still pay the price. I was never the most limber kid to begin with, and lifting heavy weights for years with little to no focus on stretching caused me issues as I got older. I took up yoga almost 10 years ago as a result, I just wish I had done that much, much earlier, Just something to think about.