Not gonna lie, that spine angle at impact worries me a bit. Might not be that sustainable long term but he's got plenty of time to make adjustments if necessary.
He has that flexibility because he is still a kid and really skiny so he can use it (for now) to get a lot of power. Soon he is going to get a bit rigid but also gain muscle so then he will adapt his swing to the new circunstancess. I dont see an injury problem, he wont be this flexible for long.
