he might not want you to meet his son until he knows you will be around for a long time. Kids get attached to people and so many times the parent won't have them meet there boyfriend or girlfriend until they know they will be together for a long time, or have been together for awhile. They don't want girlfriends or boyfriends coming and going into the child's life. My friend met her boyfriends 4 year old daughter right away they broke up 5 or 6 months later and it was very hard on the little girl.
However, it can be a red flag if you've been in a serious relationship with him for 9 months-1 year and he still doesn't want you to become acquainted with his child. If this is the case, you need to be honest with him about how you feel--you're not looking to be a parental figure to his son at this time, but you should at least have the opportunity of getting to know the child better. Does he have partial custody or visitation with his son?