Take him to the school and walk the halls. Show him the kindergarten rooms. Has he seen the kinders there? That would help. Take him to the playground on a Saturday so that he can get used to outside (find out where the kinders play.) Take him to meet the guidance counselor. Prep her as far as his "slow to warm to new situations" is concerned and ask her to take him under her wing and be friendly with him. Since you don't know which teacher he will have, he needs one person there who he knows and who will check on him here and there.
Has he seen kids get on the school bus? Take a look at this link and get some of these books from the library or from Amazon.com:
http://voices.yahoo.com/books-prepare-child-school-bus-##...
I also like The Wheels on the Bus Go Round and Round, though it's about buses in general. If you get this book, get the CD too - the song is GREAT! Children LOVE singing it.
Check this out for introducing kids to kindergarten:
http://www.cozi.com/live-simply/10-books-perfect-new-kind...
I remember loving Miss Bindergarten Gets Ready for Kindergarten. The Berenstain Bears books, though a bit old fashioned, are great too, for all kinds of situations, including The Berenstain Bears Go to School.
The best thing you can do is be excited about him going. Tell him how proud you are that he's going to be a kindergartner. Tell him how much you loved kindergarten when you were his age.
Find out from the school if the kindergartners nap during the day. If they do, you need to institute "quiet time" in his room at the same time at home. It's important that he is used to this so that he doesn't fight laying down. If he does still nap, try to get his naptime to be the same time the kindergarten naps.
Kinder readiness is pretty important. Ask the school to give you a handout of what they want kids to ideally know when they come to kinder. Don't worry if he can't do it all. Some kids don't learn to tie their shoes until they are in first grade. You can also use velcro shoes if you need to. Having him practice manipulatives such as puzzles, learning geometric figures like circles, squares, rectangles and cylinders, and learning his colors will really help. Make sure he can use scissors. Fat pencils OR regular pencils with finger grips will help him too. Teach him to sing the ABC's and count to 20. If he can recognize the ABC's and numbers, that's splendid, but I doubt that the expect it of entering kinders.
Since you have him in preschool, he is used to having a teacher and being in a classroom with other kids. That's wonderful. In kinder, he'll get used to sitting at a desk and listening to the teacher that way. He'll get used to walking down the hall in a line and following more complicated directions. At home, try to work with him on two step directions. "Sweetie, bring mommy the crayon and then go put your train set in the toy box." That kind of thing. If he's just fine with that, try 3 step directions.
This summer, take him to plenty of places where there are other children. Don't hover over him. Pull out a book and pretend to read it so that he doesn't suspect that you are worrying over him. If you can put him in a children's class, like gymnastics, that would be great too. Just stay in the background - that will help him feel that he can be independent.
First day, put him on the bus. Take your camera, and make a big deal out of the fact that you're going to take his picture. Don't ask him if he wants to ride the bus. Make it perfectly clear that this is what kinders do. (Another reason to introduce the bus books.)
When he gets home from school, hug him and say "Tell me about your day!!" Have a snack ready for him and let him decompress some before doing a little homework the teacher assigns. Make sure you read to him every day.
I think he will be fine. You're thinking about it in advance and that's really helpful.
Good luck!
Dawn