Ann Marie, your concern is well founded. I can tell that you care deeply about your son. This is good.
Since he is in high school, this is a perfect chance for him to find out just how much is too much. It's better for him to learn now than when he is in his 30's.
When I started training very very hard, it was in college, and I was running 2 times a day, everyday. Yes, I got very very tired and feeling heavy, however, the deep down strength that you can get from high mileage training can last for YEARS.
To bring you to the present day, I trained THREE times a day in June and now, in August, I can train hard and my muscles do not get sore at all.
What you should do is have his blood levels monitored. Also getting him into see a massage therapist twice a week will help a lot. I get my blood cells and iron analyzed every month, and also my other minerals such as copper, zinc, anti-oxidants, vitamins and everything else analyzed twice a year.
The body can only handle so much stress, and running is only stress, nothing else. It's the body's response to the stress of running that makes us stronger. Sometimes a lot of stress can be good, sometimes bad. It depends on just how much your son can handle. There is a very fine line between enough and too much. A lot of us on this message board are very good runners, but most all of us have made the mistake of doing too much mileage too fast.
So what I am saying is yes, let your son flog it hard and fight hard. He won't come home in a body bag, but then again he may come out of it stronger than ever. For example yesterday I did not want to train, but I know that I have to in order to achieve my goals, so I went to the track anyway, and ran a very good ten mile tempo run about 5:05 per mile. Now I am feeling great mentally, but today I will go slow.
OK all the best to your son.
Jason