It's always fun to read what everyone's son did, but it is pretty much meaningless when it comes to another young person. What I mean is that for every pitcher that went from 70s to 90s there is another pitcher that went from 70s to 70s. And those parents aren't likely to be posting their sons results on the HSBBW.
You can watch one young boy throwing 70 and his arm action, arm speed, athleticism, etc., might make you believe he will gain a lot of velocity. Another young boy throwing 70 might make you wonder how he is able to throw that hard now.
If your son is like the first boy mentioned, velocity will come with size and strength, but there is no formula for how that works. Comparisons are interesting, but each individual is different. IMO So much is determined at birth. Just think how helpful it would be if we knew what our son's true potential was while he was still in diapers. We have seen kids work so hard for years in order to throw harder, only to see others that haven't worked much at all, throw with a lot more velocity. Then again, I guess the only way to find out what your potential is, is with a lot of effort and desire, because a person's potential is an unknown. Most people never even come close to realizing their potential.