Hello,
I would just tell him its really not any different, other than separate classrooms for each subject. Just keep reassuring him there is nothing to be afraid of, and that Jr high has more to offer than elementary school as far as extra curricular activities. It will also be a chance to meet new people and make new friends. Try to play down any of the anxieties he might have, and keep a positive spin on it. He will get use to the environment in short time, and wont think anything of it eventually.

Good luck