.... you can't see anything at all in that video.
For a 19 yo that size, especially one who isn't starting with a lot of muscle mass and may still be getting taller, it is totally reasonable to gain 20 pounds that fast. If he's lifting heavy weights on short reps, eating a high cal high protein diet, and cycling creatine , he can be adding 1-2 pounds week.
I believe that NBA players don't use creatine during the season because it's not great for your endurance and because when you are use it you are carrying an extra 5-10 pounds of water that slow you down. In the off season, though, if a guy wanted to gain mass and strength, I'm sure he would use it then.
That doesn't mean James Young IS or HAS. It just means that if you're shouting it down as though it's impossible without steroids, you are super wrong!