Has he visited the school? Maybe you can call and ask if he can come and see his classroom, etc.
Tell him it is ok to be scared, everyone feels that way when they are starting something new.
I would just keep focusing on the positive and having fun getting ready for school--school supplies and clothes, etc. Try to walk him through his school day--where he will go and what he will do, sometimes the teacher will do this. Show him the school,have him meet as many kids as possible in his class, and his teachers.
I had to do this for my daughter, she gets very nervous with new experiences and the more she can see and plan out ahead of time, the less anxious and more she feels in control and can handle things. I have to do it less and less as she gets older, she has learned to do these things for herself.
I agreee with the other posters--you need to tell him it is ok and normal to be nervous, all the kids will be. You just need to model coping behaviors to him and remain un-phased by his nervousness--he is looking to you to see if he should be scared. Good luck. :)