Great analysis. I totally agree with your son's motto: if you don't need to ice, don't ice. I didn't ice either until college and pro ball, and when I did ice it was more to give the trainer something to do, rather than a real need to chill the arm.
I think we are actually seeing a lot more low-3/4 pitchers than even 20 years ago. It sort of ebbs and flows each decade. But look around the majors and you see tons of great low-3/4 guys.
One of the reasons (there are a few different reasons, in my opinion, but this is one) that velocity is so important and such a major focus is that it doesn't require a "good" scout to recognize it. The gun says 94 mph, you don't even need to know anything about baseball and you could fill out a scouting report on a kid. But take a kid throwing 88-90 mph from a low-3/4 slot and a kid's projectability becomes much more difficult for a scout. For these kids, the scout must ask, Will he throw harder, Will he get injured, Will I look like a fool around my colleagues if I recommend this kid???
Either way, I'm a fan of sticking with what God gave us and maximizing it to our ability. I'm glad your son hasn't changed his arm slot. He's still got a lot of baseball left and it will be exciting to see how much improvement he will continue to make!