In most states, children must be age 5 before starting public school kindergarten, so your son could not enroll until Sept.2011
If you want your child to enroll in some classes, he would be considered in "4 year preschool." There are some private schools, like Montessori, that may make exceptions to the birthday guidelines, but you would have to contact them directly to find out. Public schools are usually not flexible in their age cut-off dates.
I invite you to join Greatschools Preschool Parents Group at community.greatschools.net/groups/11534