Hi ! Is he able to retract the fore skin or not ? If there is difficulty in retracting the foreskin due to some degree of phimosis
, this could be the reason for swelling of the foreskin due to possible unnoticed minor trauma during masturbation
. If this and any kind of insect bite or allergy
is excluded, then may be your son may require a circumcision
to get relief. Wishing him an early recovery.