God bless this kid, he deserves all the credit for pulling that guy out of the pool. That's probably what saved the guy's life.
However...
"Patients in previous studies have cited television as a large source of their belief that rates of survival after CPR vary between 19% and 75%, whereas actual rates of survival of CPR range from an average of 12% for out-of-hospital cardiac arrests to 24–40% for in-hospital arrests."
You don't attempt CPR until you confirm that the person doesn't have a pulse. The angle up on the edge of the pool and the way the kid was doing compressions were not doing anything. Trust me, I've seen and done effective chest compressions and that shit is HARD. A 12yo tapping on someone's chest might be well-meaning, but it's not CPR
12-year-old saves drowning man, credits CPR learned from 'Stranger Things' (abcnews.go.com)