I�m not sure how much help this is; seeing how your post was put up so long ago. In an away were going through the same thing. I have a 9 month old daughter; her biological father never signed the birth certificate but pays child support. My daughter knows my boyfriend as her father and at some point if we get married he�s going to sign the birth certificate. If you have your boyfriend sign he�s saying he�s the father and taking legal responsibility. Even if you were to break up. Something you have to remember, if he sign and your relationship doesn�t work out, he can take you to court for custody (I�m not saying that he would).
In my case, when I get married my daughters biological father is going to sign all custody over to me. Even if he�s doesn�t, and my b/f wants to sign the birth certificate unless there�s a dispute, legally he would be the father in every way. Just like adoption.