18 Year Old Moves In With Non Custodial Parent

In California, upon turning 18 (not yet graduated form High School), son moved out of Dad's home and moved into mother's home.

Dad was custodial parent after being awarded full legal and physical custody of son for the three years after a 730 evaluation and prior to son turning 18.

Is Dad now required to pay child support to mother as a result of son's desire to move out and live with Mom? Father has no contact with son since he moved out. Is there any case law regarding this subject? If California law is followed in this case, it appears that the actions of the 18 year old nullify the 730 evaluation and force a Judge to order child support without an accompanying custody order.

Any help you may have is greatly appreciated.