This is a very state-specific question, so I'm just going to spill how it would happen in California. Keep in mind, you are absolutely going to have to get legal advice from a Texas attorney, since that's where you and the child are, and where the litigation, if any, is going to happen.
Paternity carries with it a duty of support, and a right to appropriate custody and visitation orders. Here, the whole legal issue of whether the husband can claim paternity depends on whether he takes some action to DISCLAIM paternity before the child turns two years old. I have no idea whether Texas has such a law or, if it does, what the age of the child is where the husband becomes the "presumed" father of the child.
One conclusion is likely to be uniform among the states: if the man isn't the "legal" father, he has neither an obligation to pay support nor the right to have any custody or visitation orders regarding the child.
The absence of a biological relationship between a man and a child does not mean, under California law at least, that there is no LEGAL relationship. You've got an important homework assignment to do, I think, and that involves talking to a good Texas family law lawyer.
NOTE: Nothing in this answer is, or is intended to be, legal advice to the reader, or to anyone else. No attorney-client relationship is created here.
You need to talk to a family law attorney immediately.
As far as anyone knows this is the father of this child. He is on the birth certificate!
This man has been acting as the father of this child.
This child thinks he is the father of this child.
Now that you are leaving, now you want to hurt your child by taking away the only father he has every known?
How is this going to help your son?
Don't you realize the damage that you are going to do to your son by taking away the only father he has every known?
And the "real" bio. father of this child - in your own words -- is a complete loser and drug addict and ex-prisoner. You want to bring this guy into his life?
You really need to sit down with an attorney and discuss this matter.
You get to choose between dad #1 and dad #2.
The court is going to make you pick one of them.
You don't get to choose "neither".