You can file  with the court a request for modification of child support based on changed circumstances since your son lives with his grandparents instead of his mother who is receiving the child support payments.You would file an Order To Show Cause  to schedule a hearing seeking modification of child support.  You should also attach a declaration signed under penalty of perjury stating the facts supporting your request of modification of child support.  The documents with a proof of service attached are filed with the court and then served on your ex.  The required court forms may vary from state to state.  It would be advisable to ask the court clerk for the appropriate forms for modification of child support.