Legal Question in Family Law in California

You'll need an attorney for this, but you should be able to file a motion with the court and ask for permission from the judge to bring your child on the trip.

You don't necessarily need an attorney, unless there are complications involved that are not listed in your question. Solano County Superior Court has a Family Law Facilitator that cannot give you legal advice, but can help you with what you need to file and how to get it filed and set for a hearing. I can all but guarantee you that the judge will rip the father a new one for trying to blackmail you out of support he owes his son. As they pronounce it in Ireland, he is a "faikin' eejit" to try to pull that.