Im not sure about the laws in california, so you will have to really check into that...
I do know however, that the reason those of you are married didnt have to have the father sign is because... You are married.
If you arent married, they have the father sign as an acknowledgement of paternity... If the father does not sign the birth certificate as well as another form acknowledging paternity, then it can be done later, but it will cost money... If you are married, this paternity is assumed unless you inform them otherwise (ie your husband is not the father)
seeing as he impregnated you when he was 17, it should not be an issue, just like when a minor (under 18) commits a crime, they are tried as juveniles, even if they dont get caught until they are 18...