Let us do a little mathematics: in the ideal conditions, outside shooting is supposed to give you a pop in jump range every two weeks while jump shot is supposed to give you a pop in jump range every eight weeks
Is supposed doesn't mean it's 100% true
Your numbers are for 18-19yo players (and I would say three weeks, not two), with proper heigh and before cross training appeared, and these are not official numbers.
Isn't he having pops in any skill or is just an outside shooting issue?