Problem with classic "bulking" for an NBA player is that they play basketball - they burn a lot of calories so it is a challenge still put on muscle while practicing and playing your sport. BBall players, swimmers, runners - all stay fairly small for this reason. And those who don't either use anabolics in the off season or are genetically gifted.
Once the season starts it would be very easy to lose a lot of the gains made in the offseason if you don't eat epic amounts of food to accompany your new muscle and possibly increased calorie burn.
I've seen guys on the juice blow up only to deflate because they neglected to eat enough food to maintain their new muscle.
With that said, he's 19, he's got a long way to go before he hits his genetic potential naturally in strength and size.