Yes, he's young and technically has time to develop, but really, how long would a 25 (or whatever) year old basketball player been working on FT's and shooting wide open 15' jumpers?
If he hasn't developed this skill to at least a passable level a by now (after at least a decade of trying to), he simply won't.
Two words for you: Jason Kidd.
two more words for you: Chauncey Billips
I don't think either Jason or Chauncey are good examples. The 2 shooting stats that I think are fair to use to compare are 3 pt% and FT% (FG% for Rondo is solid since most of his shots are close to the hoop).
Kidd shot 27% from 3-point as a rookie and in the subsequent years (still under 26) he shot 33%, 32% and 38%. His FT % improved dramatically over his career, but he was never close to Rondo's ineptitude -- Kidd shot 70%, 69%, 67%, 69% and 68% in his first 5 years.
Billups averaged about 35% (with consistency) from 3 in his first 5 years (then got better) and averaged about 85% from the line in his first 5 years (then got better).
Rondo's career averages are 24% from 3 (23% last year) and 62% from the line (57% last year).
Billups always looked like he had a shot. Kidd clearly got better in time, but was never in Rondo's league. Rondo is a very bad shooter who has not gotten better. He doesn't project to shoot nearly as well as Kidd -- never mind Billups.