I don't think his genetics are particularly impressive at all.
His size is amazing, he's extremely lean as well but that has nothing to do with genetics. Take a look at Jay cutler when he was that age - and many pro's for that matter. They looked better than this guy.
I'm not trying to take anything away from him - that's unreal for 21. And it's not because of his genetics guys, it's because he started training and juicing from a young age. Muscle takes time to put on, period. Yes some put in on faster than others, but it's not like he started the same time as you and me and grew 3x as fast - I'm sure he's been at it for a while already.