Legal Question in Wills and Trusts in California

If the only belongings your son's father had were kept in a car, it doesn't sound like the estate is worth administering (probate.) If there is anything of worth, the first question is did he have a Will, and if not, did he have a spouse? If neither, you son and his father's other children, if any, are the intestate heirs. If the father was married, his widow probably has discretion over what to do with his belongings.

You are going to have to go to court to get what you are after. You can file a petition to get the filing fees waived and then file a petition to probate the estate with yourself as administrator. Once you are appointed you can then enlist law enforcement to assist you in getting access to the vehicles contents. Short of doing all this you are going to be disappointed in your efforts.