The best thing that you can do for him is just be there for him. Let him know the differences as well as the similarities between middle and high school. Let him know that you are there for him. You also do not want to talk to much about it and worry him. Do not think it is a difficult transition that will show and pass to him making him more nervous. The best thing you can do is to show him how it is an exciting moment in his life not anything difficult. It is a time where he is growing and starting a new experience. Just make it fun and exciting by how you talk about it and that will pass to him. Sometimes we are more worried than our children and that makes them more worried, but when we do not make big deals over difficult things they usually have an easier time. Just make it a happy and exciting time.
Throughout the year keep an eye on him for any changes, so you can be there for him emotionally as well as academically. That way if any issue arises you can be on top of it as earliest as possible. Just be involved and don't ignore any signals. You can also get involved in the school if you have time if not than at least update yourself with what is going on in the school and his classes. That way you will be able to support and help him throughout the year.