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Were your son and the mother ever married?
Is their current custodial arrangement just between them or by order of the court?
Your son needs to file an FL-300 under the already established case number. If your son is represented by an attorney they can counsel him on this. If not, go to the CCC courthouse and ask for the Family Law Facilitators office. It may be called something different in your county but there will be an office set up there to aid people in filing these forms. They can guide him as to what must be included.
I agree it would be good to get it filed before he speaks to CPS, especially if the CPS report is likely to come out favorably for him.