I assume your son will go to school when he is four and a half ish unless there's a nursery class. If this is the case he will go a year earlier. So, you haven't that much time if you have a house to sell and have to find one in the chosen area. As far as I know it doesn't matter how long his name has been on the list when they begin to allocate the places - they will have a set of criteria such as living in the catchment area, having siblings already there. etc. I would suggest going to the school, having a good look round and a chat with the Headteacher, who can advise you.
Sometimes schools get a good reputation because they are good at PR. You never know the one you are nearest to now might be just as good. Remember to look at how they treat the children & what facilities there are and not just rely on SATs results. Also check out the Ofsted site to see what they say. Good luck!