You've gotten some awesome support from other mamas on how your son will be a-ok now and in the future.
A college-friend of my husband's ended up with a torsion one night and he was in excruciating pain immediately--no throwing up, no bowel issues, no stomach anything...just extreme, mind-numbing pain in his crotch. From what my husband says, that's always what he's been told to look for for himself. Doctors have to memorize thousands and thousands of ailments and they pick one that most closely matches the symptoms that are being presented. Since your son presented with stomach/bowel ailments to start, torsion probably didn't enter the doctor's mind. That doesn't get anyone off the hook, but perhaps it will help bring you a little peace knowing that everyone involved is doing the best they can for your son. Sometimes there just isn't a "bad guy" to scream at, no matter how much we want there to be someone.
I'm so glad that your boy is going to be ok and that he still has one good one left. He's no less of a man and I certainly wouldn't care less if my husband had only one testicle. I know it's a scary thing to go through and there is definitely a grieving process for the loss of a piece of him.
You sound like a close family and I absolutely commend all of you for your communication to each other (a teenager talking openly to his parents about something super-private?! Truly awesome!!) and as long as those communication lines continue to be open, you'll all come out stronger.