Interesting . . .
No other birth regs. for a 'William McKellar' or 'William MacKellar' of any surname in Victoria up to 1888.
Does his marriage certificate also name his parents as Charles Frederick Dickens and Ann Elizabeth Black?
And, if you have his death certificate, does the age tally with his estimated date of birth via his marriage certificate?
Not intending to malign the poor man, but if it is the case, he wouldn't be the first to lower his age by some years, especially when marrying a much younger bride (?).
Do you have a marriage reg. for Charles Frederick Dickens and Ann Elizabeth Black?