My question involves a person located in the state of: riverside California

I raised my grandson since a baby. He is now 15 years old . His birthday certificate was changed
when he as a child to bear our last name. As he gets older can he choose to regain his original name
that was his biological father? I went through the courthouse to have it ammended years ago.
Can I do it again. His biological father left him as a baby I see the name he want to have his last
name and his biological father on birthday certificate. It is marked unknown. Can we put this on also?
thank you