I used to play for my school and my county 1st XV and played football for my school (as well as cricket and hockey). Since leaving school (I am 34), I pretty much stopped playing team sports, though I still play squash.
I had to take a break from playing squash earlier this year and had to see my doctor as I was getting some pains in my right knee. After an examination, my Doc did explain that this was to be expected after all the sports I played as a kid, but said that I should lose weight (which I already knew), and go back to playing squash when I could.
I firmly believe that he should carry on playing - but ensure that when he leaves school, encourage him to keep up a port so that he can maintain his weight and fitness.