I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today.
It is not that the state of CA won't assist you. It is that the child support
collection arm of the state will not help you now that your son is over 18. They will simply no longer act as the collection agent for you. You will have to do that yourself, or retain an attorney to do it for you. But the state will still help you to get the money you are owed---by giving you a money judgment you can use to collect what is owed.
You need to file a Motion in the court, in the county where the support order
was last issued, and seek to convert the support arrears to a money judgment. Once you do that, you can have the CA judgment enforced in any state in the US where your ex might be.
In order to obtain a Judgment you must file an OSC or Notice of Motion which is accompanied by Judicial Council Form FL-490 entitled "Application to Determine Arrears ". Here is a link to the forms you will need:
You may reply back to me again, using the Reply to Expert link, if you have additional questions.
I wish you the best in your future,