Need advice please (sorry for long post)

Ok so I am currently 23+5 and I am kind of freaking out. The father of my child went to prison in February and I found out we were expecting in March. We are still together and everything. My problem is I am currently still married to someone else (he knows). I live in Wisconsin and he lives in another state and we haven’t seen each other or talked or anything in over 3 years. I was told that since I am married he will automatically be put on the birth certificate. How do I get around that or what do i do to get the father on the birth certificate and not the guy I am married to. I know I should have gotten divorced but I have no idea where he even is. Any advice would be greatly appreciated thank you!! And please no negative comments!!