Accepting child support

Currently, my ex-husband has Primary Physical Custody of our son, however; my son has been living with me full time (he has not lived with his father at all) for the past 8 months. During the past 8 months that my son has lived with me, my ex-husband has continued to accept my child support payments (they are processed automatically through the DA's office). No new order has yet been established.

Can you please tell me, what recourse do I have for my ex-husband continuing to accept child support payments for the past 8 months even though my son has not lived with him? I have already filed a motion to modify decree of divorce, requesting physical custody of my son but I need to know if there are any legal precidents in the state of Nevada regarding a parent accepting child support payments while the child is not living with them (even though they had primary physical custody at the time).
Thank you.