First, you cannot just start on meds. He has to be diagnosed with ADHD. Assuming that he has been diagnosed and you have tried many different options (congratulations, a lot of parents don't). Then, yes, meds would certainly be the next step. How soon the child shows improvement depends on how soon the correct dose is found. It really is a bit of trial and error as all kids are different and react to the meds differently. A good doc will go low and slow. So you may not see an improvement right away - but this is good. You don't want it done any other way. However, your doc should stay in communication with you and be changing the meds almost every week - if necessary. You do not want a doc who says, "take this and call me in a month." Lots of good guidelines here - http://www.additudemag.com/adhd/article/8942.html?utm_source=eletter&utm_medium=email&utm_campaign=March
But to answer your question. As a teacher, I have seen kids that literally go from a C to an A overnight. And as the CL here -
http://www.medhelp.org/forums/ADD---ADHD/show/175
I have seen many posts that reflect the same improvement.
As a retired elementary school principal, I think it would be a good idea to try the meds as soon as possible. I don't think it would be a good idea for him to be held back for several reasons. If the meds are successful, then the school would not have any reason to do so. But frankly, He would never have fit our schools criteria for being held back since he would have exactly the same problems.
Do realize that even if the meds do work, he still has two years of lost time and that will not be made up overnight.
If you have any more questions please post here or over on the ADHD site which I also monitor.