@worriedmother,
Was your ex a good father to your son while you were together?
Does your son love him?
If so, why would you want to deprive your son of a father. Kids need fathers even if their parents don't live together anymore.
His past conviction didn't prevent him from becoming a parent so it shouldn't prevent him from being a father. And you fell in love with him despite his criminal history so he can't be all bad. What was he in jail for? That will make a huge difference -- child abuse would prevent him from gaining custody, a pot conviction wouldn't.
Do you think your son would be in danger because of the room mate? The court can probably decide that.
Maybe you can do a shared custody where the son stays in the home and you and your ex rotate out. That's fairest to your kid.