One JustAsk member asked this question too when his child was 9 months old. Here's the answer he received: http://www.education.com/question/age-child-typically-preschool/
 
This article references age 3 to 5 as "the preschool years":
http://www.education.com/reference/article/children-three-five-preschool-years/
 
My experience when enrolling our daughter in preschool was that the minimum enrollment age varied with each school we interviewed. Many also had a requirement for potty training first too.
 
I've included a link below to The Parent's Guide to Preschool, which has a variety of resources you may find helpful once you do enroll your son in a preschool.

when i was little i started when i was 9 months so i would say he could go now if you want him to