It can be a nightmare changing schools and getting the right information to the right people. Good luck: hope it goes well for you. It's a good time to move, I think. As they have a while to get the measure of him and put him in the right sets before options choices time.
Do you have a recent 'summary assessment' with his targets and grades on from his own school? Tbh if this were me I would print up a little postcard saying something like:
"Babysparklingbrook is new and we hope he will settle in very quickly in your class. For your information attached is the most recent data of his achievement at previous school.
If you have any queries or want to advise me of anything we can do to help Babysparklingbrook settle in or catch up, please contact me using the details below."