We've done what you are doing and left our dd in school 1 and moved ds to school 2. We did it because dd was happy in school 1, ds wasn't. There obviously was more to it as well but basical school 1 is right for dd and school 2 is right for ds. Ultimately we might move dd to school 2 but not now. DS has now done a full term at school 2 and its been the right decision all round. Logisitics are difficult and its a nightmare if there is a change in plans / cancellations of things but overall its worked out and I've always had someone to meet both children at kicking out time.
I think that it is good he knows. He has time to get used to the idea and hopefully you can see his friend during the summer break and prove to him that he can still see his mate even when he is not in school.
Would put other dc for the new school and make a decision as and when a place becomes available.
I moved one and left two behind; best move ever. But the one I moved was desperate to move, and old enough to get himself to the new school (or rather to the minibus). The ones left behind don't feel left out or anything bad about it.
It took nearly a full term for DS to settle into new school.