Yotty I cannot give you any information about the WC waiting list, but my DS was on the Eton waiting list for over 2 years until he was promoted in March for entry this September, so I might be able to offer some general points.
Firstly I agree with Colleger that it is vital to get as much info about the waiting list as possible. Ring them and ask them about the weaker points of your DS's performance and areas that might need working on etc; in my DS's case it was maths, so that immediately gave us a direction to go in!
Ask if there are any other things you can do, they might suggest expanding the range of extra curricular activites, just as an example. If he is musical then perhaps consider taking up a more unusual instrument (but only if he is interested, of course, you cannot force these things).
In our experience there wasn't a lot Admissions could tell us during the first year, but they were obviously following the progress of the boys on the list, and by last summer they had ranked it into three groups, so at least we knew then that DS still had a pretty good chance of getting in.
I would recommend ringing them every 6 months or so for an update of how your son is doing in relation to the list, that's what we did and the Admissions office were always very helpful about things to try.
I wish you and your DS good luck, and do ask if you think I can help with any other info.