APPENDIX! That was my thought, too, even before I read the update. Testicular pain can actually be abdominal pain radiating to the testicles -- did they prod around on his belly area when he was there? Do any other exams to rule out appendicitis?
I've had appendicitis twice ... yup, twice

. First time ... a million tests, they couldn't figure out if it was or it wasn't, sent me home, and I got better. Second time ... they yanked it out! (an appendix can actually "go off" several times before becoming severely infected -- who knew?)
Appendicitis is the most difficult condition to diagnose -- tons of folks are sent home wihtout it being diagnosed properly (in my case,and maybe your son's case) and on the other hand, tons of folks get it taken out when it is fine. There are some tests they can do to be more sure (they need a white cell count and a CT scan for sure) but it's still damn hard to diagnose.
Mid abdominal pain near the belly button is classic for early stage appendicitis. Usually ti will eventually shift over to the right side, but for some people it never does. Also everyone has an appendix in a slightly different spot -- some people's appendixes are even in the middle or on the left! The moderate to severe pain (even if it's in the middle), vomiting, nausea, and positional-related pain are classic appendicitis signs. Fever may or may not be present, but white cell count is usually elevated.
In any case, he needs to be seen back at the ER ... now. And this might turn out to be a virus or something (fingers crossed!) but if I were you, I'd be fairly aggressive about making sure he gets that CT and any other test they can throw at him, becasue your little guy could get very very sick very fast if they misdiagnose this.
Best of luck to both of you and update us when you know more. Fingers crossed that it's a weird virus, but if it is his appendix and it's in the early stages -- not to worry, he'll be in and out of there and fully recovered from surgery in no time flat!