Thanks, but if you take a moment to think about it... I asked my buddy to shut down one of the tanks.. If I want to know which one was off, I can just press the purge button on the second stage from each tank and I will have my answer in 2 seconds.
I was asking him for information that I should have been able to determine on my own.
In my technical training, this type of scenario (massive gas loss from undetermined site behind my head) was common. The first and most basic rule for the buddy coming in to help was never to turn off the gas the "victim" was breathing. We were taught, as we approached, to ascertain which reg the diver was using, and if he was using the one that needed to be shut down, to signal him to switch and confirm that he had done so before shutting the valve. I don't know if your son did this or if he even thought about it, but since it was your pony reg, it wasn't a big issue, until his attempts to tell you which reg he had shut down caused you to try to switch!
Having a standardized and practiced set of signals for dealing with this type of situation is useful. We tend to use a finger pointing to the piece of gear that is no longer operational, followed by the middle finger salute