I have sole custody of my son who will be 18 in Feb. He wants to live with his mother who doesn't have a stable place to live. She is also in arrears by at least $1,700.00 in child support. My son is still in high school at least until May... "

What does the support document say? You are asking about child support and you are asking about custody.

What is your question? They are unrelated in the eyes of the Court - custody does not hinge on visitation.

His mother could go to Court and request custody. The Court will determine what is in his best interest UNTIL he is 18. Then (in most States) he's an adult and can live wherever he wants.

Which State (or Country) are you in?

We live in California the question is , he wants to go live with his mother when he turns 18 in Feb. but we have sole and physcial custody of him since he was 8 years old and he will not be out of high school until May of 2013. Can he move out when he is 18? Thank you.

Yes he can move out at 18. And unless the change is made legal she will still owe child support to you for the child until he graduates High School.