How much exercise is too much for an average fifteen year old boy?

Varies. For a healthyb15 y.O boy there is no such thing as too much exercise. As long as you are not having continuous pain from your knees, hips, ankles or muscles there is no restriction. Do as much as you want, but do no harm based upon how you feel each day.
Individualize . Every 15 year old is different, especially with respect to their maturity. Thirty minutes of aerobic activity five times per week is good. Two of those work outs can be strength training. At his age focus on technique and a balanced work out. If they are alreay in a school sport that is enough. One sport/one team per season, short period of rest inbetween. Sleep at this age is crucial.