VERY scary video, so glad it all worked out and that everybody is safe.
I think this was a case of trio diving, right? I know that can be challenging, as that's how we go--mom, dad, and (now college-aged) son. We've had some difficult times sticking together, followed by some harrowing moments, so I am totally sympathetic.
But I'm going to ask a (perhaps) stupid question. Why doesn't anyone have/use a tank banger or quacker? It seems like the diver in distress was trying to get attention by screaming through his reg, and that doesn't work so well, as we all know. Or have I misread the situation? I may have, as it was very hard for me to watch this video, expecially when he started screaming. Ouch.
I did notice in Cozumel that very few people had or used noisemakers; I know they can be bothersome, but as long as they are different from the one used by the DM, I believe they should be considered a safety essential--just like a computer, sausage, light or whistle. Set up a system ahead of time--this many honks means "hey, look at this!" and this many honks means "Where the hell are you?" and this many means "HELP!"
I'm not going to MMQ this, because, as I've said, "been there, done that." As for not inflating his BC, I'm guessing he didn't want to waste time and just wanted to get his son down. I would have done the same, but then it takes me a while to get down, especially once I've been up and my tank is closer to empty than when I started. Anyway, dad needs to do what he needs to do. #1 job is protecting his son.
Okay, technically not doing the MMQ thing, but hope you will consider the suggestion for a noisemaker, if you haven't already.