What Giftodd advise you sounds great, but I would like to advise you something for the here and now, since I've experienced the same with some of my kids. Is there anything that you can interest your son in that other kids his age also like? It could be Legos, or bagukan, star wars, or sid the Science Kid, anything that the preschoolers play with and you can interest your child. Once you find something, speak to the teacher and ask if he can bring a toy/book (could be a library book) on the subject. These items could serve to open up conversations with the other kids. It sounds crazy, but I advise you not to let your preschooler know too much on the subject! He should know more or less like the other kids. As he becomes interested it may become hard for you to stop him from learning more, but at least he'll be at more or less at the same level as the other kids at first.
Anyway, this helped me a lot with my kids. I admit that now my nine year old is beyond that...but at least he does like Harry Potter and that gives him a subject to talk about. He can practically quote the books verbatim, but that's another story...
