I say, good for him, he got to I'm sure a goal of his.
I also say, so what... PADI Master Diver isn't a real accomplishment (I'm not) All I'd have to do is go get 3 more specialties, and pay for the C-Card... it's not very hard... 50 dives is easy in a year, as is getting through all the classes. And PADI's Master Diver is another C-Card that is all about the money... to get a C-Card, which just says you have 5 specialties, is LAME! Now if it intitled more training, that's a different story... maybe similar curriculum to Dive Master, only on a non-professional level, not able to help with classes, etc... but at a similar level of training and knowledge.
Would I dive with him... sure. I'll dive with anyone once! Would I want to dive with him a 2nd time... depends on how good a diver he is. I dive with lots of people who annoy me as a diver... buddies who seem 100% oblivious to me, and I have to do all the work as a team... that's who I won't dive with again. (among others)
Could he handle a rescue situation... I honestly think he would have some trouble... now in the moment, if he had adrenaline on his side, he might handle it... I know it was hard for me to haul some of the big guys in my rescue class out of the water, and I'm 6' 0" and 170lbs (kinda a wimp though).
A bigger question if you were to show up on a boat to be buddied with him, is why am I diving with him, not his parent or a DM? IMO a 12 year old, is most likely and should be diving with a parent, or close friend of the family... not a complete stranger (course this is all hypothetical, but this thought would run through my mind in this situation, unless he was a family friend of course)
Enough from me. Deffinately a controversial subject.