My husband had this same pain a couple of weeks ago, he went to our family doctor who couldn't find any problems by touch. So he was sent to have an ultrasound done of his testicles and they found he had tubular inflammation. He is an ex-comp swimmer and is now very active, the doctor just told him to take it easy for a little while and wear boxers until the pain goes away.
And not to scare you, but boys/men from the ages of 15-35 are at a higher risk of Testicular Cancer and I would ask your doctor for an ultrasound to be done on your son. But for all you know he could have the same problem as my husband and he needs to give the speedos a break for a little while.