Hi Zak:
There are several things to try/do now.
Has your aunt tried to order Charles' death certificate? The information on it should be somewhat accurate, but if Charles' relation didn't know the answers to some of the questions, there may be "unknown" as replies. This is a site where I would suggest ordering the certificate:
https://www.acgov.org/auditor/clerk/death1.htmThere may be less expensive ways (sites) to order certificates, but I start with government sites as I'm not a trusting person.
Please read everything thoroughly, as I've never ordered a California certificate and I don't know the process.
I don't remember if Charles' marriage has been mentioned. Would a marriage document have witnesses' names? Perhaps a relation from Charles' side could have been a witness? When ordering a copy of a Canadian marriage document from a church for one of my ancestors (husband and wife, so two ancestors)
included on the document were about six witnesses - all family members.
If Charles was born in May 1864, you may want to expand the California search to include 1863 and possibly before. For decades I believed my great-grandmother to be an only child. A few years ago, we found out she had at least three sisters. A few months ago a very kind RootsChatter found a possible sister of my great-grandmother and it now looks like she had at least 8-10 siblings (some died when they were young). I would not be surprised to find out Charles had siblings.
If you aunt has any stories or knowledge of Charles, the tiniest bits of information may be clues.
As a last resort, you could try following up on the marriages from the early 1860's to find the families on census returns. I don't know if all California counties were included in the marriage records, though. Also, if his parents were born in England, there is always a chance that they were married in England, or on the East Coast of the U.S.
I look forward to hearing if your aunt has any photos or knowledge of his life (no matter how insignificant it may seem).