If there is a court order. And there is a monthly amount then he will be in what's called arrears.
What you need to do is have that order enforced. You need to get a lawyer or someone there to help you if possible. You might also be able to do this yourself but it is your mom who is the beneficiary.
California Department of Child Support Services > Home
You will need to explain in detail and provide all proof as they ask you to do. They can help you collect it. It may be a matter of jurisdiction and it may need to be transferred. But it is possible.