It depends on your situation, and on the duration of your Ex’s obligation to pay support. You see, the job of the Family Responsibility Office (FRO) is to enforce existing Court Orders and separation agreements that impose child support or spousal support obligations. The FRO’s enforcement authority is directly driven by what either of those documents say in your case.
So if the governing Court Order or agreement says your Ex’s support obligation ends when your son reaches 18, then the FRO’s involvement will cease accordingly. If your Ex is obligated to pay even after your son turns 18 (for example under an agreement where you and your Ex agree to fund his post-secondary education), then FRO may continue to be involved until the support duty ends.