He has a full western grip, a lot of racquet head speed, and an extremely heavy windshield wiper. Kyrgios has all of that too except with a less extreme grip and probably slightly less racquet head speed. I don't think it's really a new evolution in tennis though, people have had heavy windshield wiper forehands since the 90's.
Just a very loosey loosey arm. I think tennis coaches and players are more and more figuring out that a loose arm is the most important thing in technique in modern tennis. Funny how the technique is so much different than when they played with wooden racquets. I quite like the loose arm shots that Kyrgios and Sock have. Makes for some pretty effortless huge shots and more explosive tennis. This shot in particular isn't that aesthetic though, but Sock's game in general isn't..
I disagree about his career by the way, I think Sock will make the top 10 and win some (big) tournaments here and there. He has a big game and once he improves his court movement a bit, he'll be a threat to anyone. I'd focus more on my singles career if I were him. Winning doubles GS is cool and all but this is the age where he has to really step it up and he can't be dividing his attention like that.