I see you rated the service "bad". I would ask you to consider removing that since it was based on you not being able to view the post. That rating affects me negatively and I hope you could wait until the end of our conversation and rate me on my demeanor and understanding of the issue. :)
It's hard for me to say whether the birth certificate was valid. If it was filed with his name on it, yes it would be valid.
I think your bigger problem is that if they were married at the time the child was born, the child is presumptively his under the law. (I know that you say the current marriage is not valid, so that issue would have to be dealt with before dealing with the legal presumption). That's a bigger issue.
If they were not married at the time of the child's birth, but have since married, and she placed his name on the birth certificate, that grants him parental rights
over the child. You really need to speak with a local family law attorney about establishing your parental rights OVER the current husband's rights. This is going to take more than just a name on a birth certificate to fix. You'll need to have a paternity test completed and then file an action in your family law court to have yourself deemed the legal father. Realizer however, at that point you will also be obligated to provide support for your child.
If I can answer anything further, let me know. Remember, I'm here to help and your satisfaction is guaranteed. If you don't have any questions and you valued my time, I'd ask you to consider rating me positively.