It all depends on the preschool capabilities of the public schools in your area. There are some schools that offer 2 years of preschool and let the kids start at 3 others don't allow them to start until they are 4. Different states have different age requirements. If your school needs your son to be 4 before Sept. 1 (or a date prior to his birthday) then unless they also offer a 3 year old class he will not be allowed to enter public school. Check with your local school to find out what they offer and what the age requirement is.