There are many problems with your attempt.

First, ignore the website provided by the other, that is for Los Angeles County. There is no shotgun response for your inquiry.

To obtain a copy of a birth record in CA, IF he was born after 1905, you could obtain it from the CA Department of Public Health:
http://www.cdph.ca.gov/certlic/birthdeathmar/Pages/default.a...
---But, since this was in 1889, you would have to contact the "correct" County office. If you are sure he was born in Riverside County or another county, use this site to find the correct county office. But, this is for normal birth registrations.

BUT: If you are requesting a copy of a birth record of a child who has been adopted or had a legal name change, see:
http://www.cdph.ca.gov/certlic/birthdeathmar/Documents/Adopt...
IF the original name/birth name was sealed due to the adoption, you may have to get a court order to have it unsealed.

Then send your written request should be sent to the State Department of Health Services, Office of Vital Records - M.S. 5103, PO Box 997410, Sacramento, CA 95899-7410, the telephone number is (916) 445-2684 . There is a $16 charge for each copy requested payable by check or money order to the office of Vital Records.
"IF" there was a birth record prior to adoption, CA law may have it held in secrecy due to the adoption. If there was a "substitute" birth record, it would show the names of his adoptive parents. Obtaining a birth record would be very difficult, especially since you don't know where he may have been born.

Your search in this matter is HIGHLY improbable of success.