Is your son still a minor? Who is the child living with? Is there custody/placement rights in place? If you have a court order stating that you have placement and custody of the child, show SS these documents so that they may revert payee back to you.
There are several reasons the SSA might agree to a change in rep payee...living situation, court order, custody, questions of handling the funds properly, input from your son if he is over 18, among others. This typically doesn't happen without significant review by the agency. You might need an attorney to assist but this is not a case for contingent fee. You will be charged hourly for their services.