Read about inguinal hernias. They're very common. One of the most frequent surgical repairs there is. My husband and son have both had them, which, to me, means they may run in families. When guys are young and their testicles descend, they follow a path called the inguinal canal. That pathway sometimes leaves little weak places in its wake where, with strain, little sections of bowel can poke through. That can cause pain with straining and, if the bowel gets strangulated, it can cause real problems.

Sure, it might just be pain from a pulled muscle or simple straining of some other kind. The point is he can't know until a doctor checks, and he doesn't want to risk having it be a hernia that gets strangulated or gangrenous, especially back there right behind his package, when it's so easy for a doctor to evaluate.
Inguinal hernia - Wikipedia, the free encyclopedia