You might want to look at a montessori instead where the children can develoop at their own pace and are often grouped according to abilities and not necessarily age. When he turns 5, he'll be more than ready for Kindergarten. I was pushed ahead, and honestly, I think it was a bad thing. Intellectually I was more than ready, but not emotionally. The girls were just that much ahead of me when it came to things like boys, etc.
Your son is shy and might do well staying with his age group when it comes time for Kindergarten. My son is very outgoing, smart and big. He was accidentally put in the class ahead of him (long story). He did fine, but he's done much better with his peer group since then. He's also been happier. At his age, and your son's age, a year makes a BIG difference. Most of the work in Kindergarten is stuff that the kids already know so your son won't be the only one in that position. It's really the social environment that's the big thing. I would vote to let him stay with his peer group at least starting with Kinder. Ultimately, you'll know what's best for him emotionally but I just wanted to throw in my experience. Good luck!