A half marathon is 13.1 miles. That can be a safe distance for your son if he runs a comfortable pace that would allow him to talk to a running buddy, and if he runs free of pain. That pace is known as LSD (Long Slow Distance). His body is still growing, and he should avoid putting significant stress on his body from running. In working up to the 13.1 distance, he should follow the 10% rule and run heavy/light. Here is a reference about the basics of running.