A.V.
My SS had to change in 6th grade b/c his private school folded. I was also a private school kid who changed to public in 5th grade. I think if it's possible to start him at a grade level where other kids are new (so in our district, middle school is 6-8) so he's not the only new kid. I started a new district in 5th and then another new one in 6th. 5th went better b/c everyone was all mixed in from various schools and 6th was rotten b/c in that district MS was 5-7 and I was a year late.
Do talk to him about changes. For example, class size, keeping track of his stuff (having lost things be lost forever was a huge shock to me) riding a bus if he doesn't now, etc.