What OldBugle suggested --- they call it the Madonna-Wh0re syndrome -- was your FI present when you were in the delivery room?
A sex therapist and mental health professional can help.
1) It's normal for a couple's sex life to change over time. You're no longer in the honeymoon stage.....you've been together long enough that the novely has worn off, and that's pretty normal.
2) Was this pregnancy planned? Perhaps your FI isn't really that into you anymore, and is only sticking around because of the baby. I mean, it sounds like you got knocked up shortly after you started dating.........maybe he would have broken up with you a year ago had you not been pregnant with his child.
My money is on #2. He's engaged to you out of obligation, but he's not happy about it.
And the OP is how old, 18? Wow...being a father this early on into a guy's life can be very very life changing. Most guys his age are out sowing their wild oats, going to college, etc.
You said that before the baby the sex was great? "Great" yeah, because what 17year old boy ISN'T horny? And being that he was 17 and very inexperienced, I can't see how he was "great."
I was also thinking that perhaps -- and it's crappy and poor timing, what with a kiddo that just got into the picture -- that it may very well be that the relationship is over. He may not know how to let you off the hook gracefully or he's waiting for you to end it, As I told a poster on another board, guys will freeze you out and give you the cold shoulder and make themselves otherwise scarce --- only when you insist on a declaration of hate will you get it -- and then the relationship will officially be over.
(and this is why kids minus marriage are not a good idea. It puts a strain on the relationship, whether or not the pregnancy has been planned -- and at age 18, doubtful if you and he planned this pregnancy at all --- and I also strongly suggest birth control for you. I am not trying to bust your balls or ride you or flame you but wow, if you are going to be sexually active you take precautions and use birth control -- and a condom at all times. Adults who are sexually active are responsible. Ever heard of HIV, AIDS and STDs...and unplanned pregnancy?)
You need to talk to him.
Ask him straight away if he is still interested in continuing a relationship with you and be prepared for the answer. If it is no, do not bother to make plans to revitalize the relationship or stay with him -- very very few guys and girls that are 18 or late teens are ready for a full time partnership, with or without marriage. If he is evasive or doesn't want to talk about it or your initiation of the subject results in an argument, call it quits and move on. Discussing it any further will get you nowhere.
See an attorney to get child support and visitation straightened out -- don't let him off the hook scot free --- go to legal aid or call yoru county bar association to find a low cost attorney who can be of service to you in that capacity. Wishing you luck.