First and foremost a HUGE CONGRATULATIONS that it is not cancer and that you can get back to normal soon.
In my case the doc told me there was a slim 1% chance that I did not have cancer but not to count on it. So I prayed for it but alas it was my Dark Passenger and hopefully the I/O booted him forever more. That being said, I do not believe that I would have been angry at all if it turned out not to be cancer and I had gone through the I/O "for nothing". I would have celebrated.
That's just me and I am 39 with 3 kids and not planning on others, nor is my wife planning on allowing me any girlfriends . If I were 15, my take on it may be a bit different but at the end of the day you are healthy and alive and just starting your life at 15, so sue them for having missed the "replacement" teste, but personally I would be very happy with the outcome.
Now from a more legal point-of-view, if the Doc took reasonable steps to diagnose your son and had convincing evidence that the diagnosis was TC and also had/has reasonable experience on the subject matter, I think you'll just be making money for your attorney. It all comes down to how reasonable were his actions based on the evidence and his experience. Now, if this is a repeat performance for the Doc and he does misdiagnosed I/O's regularly, then you are facing another situation, but you do have to prove it.
Whatever you choose to do, just celebrate that your son is healthy and cancer-free and will live a long life and hopefully never have to take out a membership on this site.