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?
His player is 6'9 and 20 years old. Given that range is extremely height-sensitive, these two alone may be adding a lot of time to 1 pop.
So his case is pretty far from ideal conditions.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."