Hopping on a plane without a plan and doing some groundwork will not be successful. First, you want to identify what resources are available to assist your son in San Diego, including the Public Guardian's Office, the local legal aid program and resources for persons with mental disabilities such as the local chapters of the National Alliance for the Mentally Ill (NAMI) and California Alliance for the Mentally Ill (CAMI).
You want to review California's guardianship rules. There is information about California guardianship and conservatorships at the California Judicial Council website.
I also recommend that you contact an attorney or organization in the San Diego area that handles probate and mental health matters to assist you with this matter. Good luck with this.
This is not a substitute for a consultation with a mental health disability rights law attorney or a probate attorney in the San Diego, California area.