My divorce states that i have custody of my 3 kids but my son decide to go live with his dad...he pays child support for all 3. Now that my son is no longer with me how does the child support work?

Just can't go change living arrangements at will here, need the courts permission to do so and has to be approved and then the judge has to altar child support to fit the situation so right now as far as the courts are concerned, you still have all 3 kids and he has to pay support as normal til courts approve or both could be in legal trouble