Change name of 4 yr old son

[mscastro]

I live in California. The father was never named on my 4 year old son's birth certificate and I gave my son my last name. Now, I would like to change my son's last name to his father's - his father is amicable - and name him on the birth certificate. How do I go about this? I was told at one time that his dad would have to adopt him. The father's name is left blank on the birth certificate.