Not sure if you're still checking these message boards, but I'll offer my advice in case you are.
Running a half-marathon at this age isn't going to do any damage. Serious half-marathon training might (burnout being the most likely injury), but the race itself won't.
I'd recommend you tell him that of course he can run a half-marathon, but first he has to run a 10K race. See how he handles that. If all goes well, have him try a 15K or a 10-miler if there are any in your area. If not, go ahead and let him jump to the half-marathon from the 10K. Making him take it a step at a time and earn it will help him develop good habits for running & life in general.
Just don't let him get too serious about training yet. Stick with the training he does for soccer, and add maybe one long weekly run of up to an hour, nothing more. That should be fine.