Never married, amicable, no custody agreement or child support.

My ex an I have an amicable relationship. We have a 7 y.o. She moved a block away to keep him in the same school and close. She has a busy life and I have the kid 4 or 5 days a week usually. We were never married and we have no custody agreement. We lived together 7 years. So far, she has not asked for child support (why would she, I usually have the kid.) I'm afraid out of the blue, her life changes (she gets married or a good job across the state.) She could suddenly move with my son and start asking for child support. How do I go about getting a custody agreement for shared custody and how do I not get stuck paying back child support when I am currently already paying most of the child care expenses (after school care, lunches, feeding and housing when he is with me, clothes...etc.)?