Although each state has a cutoff for when they expect a child to enter kindergarten, parents are left with the ultimate decision. If your child is currently attending preschool, the first step would be to talk to his teachers and ask their advice.
The ultimate goal of preschool is to prepare a child for kindergarten, but that preparation does not necessarily focus on academics. The more important skills a child needs in order to move up are social—understanding others' feelings, knowing how to share, participating in interactive play. Your preschool teacher should be able to give you an idea as to where your child falls on the continuum of his peers in this area.
Additionally, it is important to realize that if your child is having difficulty socially or academically, special education services are often more comprehensive once a child enters kindergarten. Therefore, if you are concerned that your child may require special education services, it is a good idea to seek evaluation early on to find out what services are available in the public school.