J.H.
We were in the same boat last year. It was a difficult decision because others his age went to kindergarten while we opted for preschool. I have never heard anyone complain that they regretted holding their child back, but I have heard people complain that they would have wished they did. We felt it is better to have the oldest child in the class rather than the youngest, especially with boys who tend to develop slower. We felt it would help gain self-confidence with our son, who was also a preemie.
Either way, if you inquire at your school they will give you a list of requirements that the child is expected to know prior to entering kindergarten.