Hey, it sounds like to me that it may be a hernia, or perhaps epiditymitis, which is inflammation in the epididitmis. (I may have spelled that wrong, my apologies) I actually suffer from a chronic form of this which occurs every few months. The abdominal pain he is experiencing could be similar to the feeling you get when you get kicked down there. . . Very painful, and shooting. The epiditmis can also become swollen to to a UTI, just a few things to possibly bring up to your doctor. They may have to do an ultrasound on his testicles, embarrassing, yes, but absolutely necessary. Best of luck!
http://www.goldjournal.net/article/0090-4295(81)90012-1/abstract
This article says exploratory surgery is in order.
How old is your son? I would take him to another urologist. This is nothing to mess up. Time can be critical. What if this guy was wrong?