Without seeing him, I can not say for sure. Calacaneal apophysitis is noted in children of this age. Until the growth plate in the heel closed, this can occur when overuse occurs. Common entry level treatments can include ice, heel lifts, orthotics, stretching, taping, orthotics and REST!!! Many child athletes and their parents dislike hearing this part, but it is true. Very infrequently, immobilization is indicated.
I recommend you take him to a podiatrist for further evaluation.