You committed a crime (fraud) when you knowingly signed a birth certificate declaring that to be your child when you knew he had a biological father who was in jail. And until a court rules that you are the daddy (through a dna paternity test), you are not the legal father, even though you signed that birth certificate. The courts may permit you to pay child support, they may even demand it, but unless you can prove you are the biological father, you will never be awarded visitation, much less custody.
Also, the mother of the child can deny your paternity at any time by calling for a dna test. You would have gotten attached to the child, perhaps even paying for his expenses, and then when she found a new man, you get kicked to the curb with no rights at all. Not only that but the father of that child can report you to the authorities for falsely signing a birth certificate when you knew he was the father.
Get a lawyer and get your name removed from the birth certificate. You know you have no right to that child so you had better get your name removed now before they make you pay support anyway, while denying you any rights over the child (and yes, that has happened before). Good luck.