It looks like a strained muscle due to overactivity.Apply some warm compresses on that area and give some over the counter anti inflammatory analgesic(pain reliever) like ibuprofen(motrin) or diclofenac.Ask your son to abstain from overuse of the muscles and do some tretching exercises before.
If there is no relief from the symptoms after analgesics,then the other causes of such psin like inguinal hernia,appendicitis,bowel disease etc have to be investigated.
Hope it helps.Take care and pls do keep us posted on how your son is doing.
I also thought maybe from over straining muscles not used so strenuous recently.I know he definietly stretches out before practices/games. We'll try the warm compresses/Motrin and see how he does.Thanks for another opinion!