1) If the father / son pair have not been diving a couple years, I venture they prob had to rent equipment. Didn't the dive shop check the pair out for experience and give them proper advice?
Nope. No doubt people will try to find fault with someone, but it is not the shops responsibility unless they demonstrated some very strange behavior when renting gear.
2) Going deep for whatever reasons, I'm pretty curious if the pair did a dive plan. Like perhaps the max depth for their dive given their certification level.
Plan or no plan, going to that depth proved to be a bad decision for them. The lesson we should learn from this is that accidents can and will happen and at that depth, the consequences are dire. Until the son reveals more information, all we can do is speculate and I happen to believe they planed a deep dive then Nitrogen Narcosis changed the plan and the tragic outcome.
3) How do you lose your grip on your buddy when sharing air?? Unless maybe panick set in, combined with an uncontrolled ascent it might be easy to be separated quickly enough to prevent a re-join.
Inexperience, Nitrogen Narcosis, Panic and OOA changes even the best of plans.
I don't mean to be a hardnut on the kid. Terrible tragedy to suffer... But it always seems to me that tragedies are a result of a series of mistakes. Hopefully the more mistakes we identify and prevent the less such accidents are repeated.
The son is going to have to live with whatever happened. His life has been tragically changed forever and will need significant help through this. Divers need to take dive planning more serious. All to often, diving deep is the goal and I’m just as guilty. When I first started diving, I could not wait to go deep. So deep I went. Fortunately, I survived but it was just dumb luck.
Here is a warning to new divers: Don’t go deep, you won’t find anything down there but colder water, darkness, narcosis and the pleasure of watching your air get sucked out of your tank at an alarming rate.
If you must (and we know your ego requires it) go deep, do so only with an instructor or very well qualified diver, have a good plan and know your limitations.