If you honestly think his academic and social interests will be better served in the new school, I think this is a very good time to make the change. It's a natural point for that to happen as he's moving on to high school either way. It's not like he's leaving town, so any friendships from his previous school can be maintained, especially with cell phones and Facebook and all that, with weekend get-togethers and the like.
I don't think you're wrong in making the final decision, given his age. Changing schools is a big thing, yes, but if this new one is that hard to get into and he worked for that, I think he should go.