Petition for Appointment of Successor Custodian

I think I've narrowed down my questions this time:

I've looked all over the internet, and even went to the Fresno Ca, County Clerks office, and still can't find the right form to Petition the Court to be named Successor Custodian of some bank accounts my Son's Grandmother set up for him before she died, because she didn't name a Successor Custodian to the accounts, and he's only 13. If he was 14, he could name his own Successor Custodian.

What I need is a California Superion Court form to Petition for Appointment of Successor Custodian -- or something close.... some people have told me that I need to file a Petition for Appointment of Guardian of Minor (which I've found), but I'm his Father, and have Sole Legal Custody of him, and the Legal definition of Guardian says, "persons other than Parents, assigned by the Court", so I don't think that this form is what I'm looking for.

Any and all comments, information and/or suggestions will help greatly!!!