I would strongly suggest mentioning the school change to him after he has completed this school year, and for a number of reasons:
First, kids live in the present. That is to say, they are by nature in the here and now, so when we distract them from the present to tell them of upcoming changes, this can often cause children anxiety. Most especially in a situation like this one.
The fact of the matter is simply this: while adults--with all of our life-acquired knowledge-- often do well with knowing about changes/transitions way in advance, children do not have the life-skills and experience to know how to manage their apprehension or anxiety about change. New is simply NEW. Depending on the child's temperament, they may really need to experience that new thing ( a sport, a new school, a new teacher) before deciding they are okay with it. So, that is something to consider.
Now, also consider that if you bring up this change during the school year, he may very well ask :WHY. None of the reasons you are changing schools would be, at this moment, flattering to your son or the school he attends. Telling him that this school is 'too hard' or developmentally inappropriate (which, from your description, sounds awful) may very well have the affect of sabotaging him for the rest of *this* year. You want him to be confident in the friendships he's made for now, you want him to put forth his best effort. So, he needs to think that his current school is *where he belongs* for now.
In the summertime, you can share the good news with him that you found a school which you think will be a better fit for your family. That there will be other kids there, some of them will be new as well. Since you live across the street, there's more time to linger at the playground and make new connections. There a lot of great positives in what you describe.
So, please, save those positives for a time when it will cause the most happiness and the least confusion for your son. The reason-- I believe, anyway-- that parents often cue their kids too far in advance of upcoming changes (new babies, new schools, new caregivers) is that the *parents* have anxiety about the changes. I've actually written more about this if you want a bit more on the topic (I'll put the link in below--the article is on how to share transitions with our children) and I really hope you will hold onto this good news a bit longer. Congrats!
http://skyteahouse.blogspot.com/2012/07/present-tense-let...