Hello,
In my opinion your son is suffering from retractile testis
Retractile testicles are very common and it merely means that although your testicles hang normally in your scrotum most of the time, they are pulled up by certain muscles into the groin area in certain circumstances. Swimming, especially in cool water, will almost always cause this, as the natural tendency of the muscles in the skin surrounding the scrotum is to contract in the cold. The cremaster muscle pulls the testicle upwards and in some men into the inguinal canal that lies just above and behind the crease at the top of the leg where it joins with the abdomen. This happens too when you are nervous or anxious as a result of stress hormones, as well as during sporting activities. During physical exertion blood flow is diverted to the exercising muscles and away from the genitalia. Again the testicles are pulled up as a result. During sport, the scrotum contracts to protect the testicles from harm, and in fact, retractile testicles have a protective value in contact sports for this reason. Professional sumo wrestlers in Japan actually massage their testicles into their inguinal canals deliberately so they do not become injured during wrestling. Testicles normally, and naturally, retract during sex, especially prior to ejaculation. Whilst your condition is normal, it can sometimes be uncomfortable. You will find that wearing warmer under clothes will definitely help as your scrotum will relax just as it does in a hot bath. You could also wear a sporting jock strap rather than ordinary pants, as these are more comfortable to wear with your condition. Please check this link
http://www.netdoctor.co.uk/ate/menshealth/205150.html
Doctor (MD)
M.D. (Radio-Diagnosis), Experience of more than 25 years in General Practice and Radiology.