The custody battle I am going through has been the longest, hardest thing to get through in my life. we have been doing this for 13 years. it is still a constant struggle with his dad.
Another thing I forgot to clarify - the more involved he is from the get go, the more reason the courts will have a reason to keep him involved.
Cut and run. It's not nice, but the baby needs you, not overnights with his aunt. Because that is what will happen, she will help him get custody and end up babysitting for him when it's his nite. I have seen this so many times in my circle of friends, it's not just my situation I am speaking from.
Cut and run. He has made it easier for you by being in jail. Maybe find a job somewhere else, or enroll in school, start financial aid. Educate yourself on the different counties' family law courts, and try to establish a residence and therefore a case in a more mom friendly county, if necessary.
I know I sound dramatic, but I am just telling you the things I wish someone had told me.
This is not only for 18 years, it is for life. just think. for 18 years you and he will have to deal with each other's choices, relocations, job schedules, spouses, new children.... you need to get the upper hand out of the gate to keep your freedom from him. He (or his family) can control you through that child, and that effects everything for you.