My girlfriend and I have been together 5 years. She got pregnant and we had our first child in May. 5 months later, I find out I'm not the biological father. The biological father wants to be in the picture everyday of our baby's life. And he wants to give the baby his last name and wants to take me off the birth certificate. My girlfriend wants to allow it. I'm worried he's taking the first steps to have the option to have custody one day. Do I have any rights in this situation or did I just lose my family?

Ouch.
You REALLY need professional help with this one and it WILL be a serious fight, so be prepared.

1) How did you find out that your not the bio father?

2) Did you sign the birth certificate or aknowlage parenity ?

Basically you have no rights here. If you were married you would have been the presumed father. But even that would not have be enough for such a young child. The bio father needs to go to court to establish paternity, but, depending on local laws, he probably can removed you from the picture.