It really depends on genetics, their neuturing history, and yes, to some degree diet.
The thing you should keep in mind is the Shelties you see in the ring have been groomed and trimmed for show, so don't try to compare your dog to show dogs.
Most dogs will grow into their coat by 2-3 years. Though I've heard stories as late as 5 years. Both my boys are 4 year old neutured males, and I'm still waiting...that being said, both have had brutal groomings which may have delayed their hair growth as a result.
Can you post a picture? It might help us assess where he is at.....
TTFN