Dump him. The drama level on this situationship thing seems to have exploded past the point when this question should even exist. If you stay with him, that means you will be in a relationship with a baby, a baby mama and a guy who has already demonstrated his low responsibility level in getting someone pregnant... and who will never be able to give you what you want in the relationship because you can't be a sole priority anymore. There are other things a new parent needs to deal with that will smother a relationship. There is a reason why people plan out having kids because its hard on everything: money, bills, stress, the relationship/marriage. This isn't your problem to deal with so why would you put yourself through that?
As you explained, he didn't cheat on you and yet you are now the side chick. Let that sit for a while. 0.o
The other woman and her pregnancy needs will always come first. When the baby is born any feelings of envy, regret, anger or frustration you may have with this situation will become greater.
He might tell you that everything will be okay and you might even convince yourself that you staying with him and helping him pull through this will make you a super great person and the ultimate helping hand but you are kidding yourself. If you want to be nice and help then give him some pampers after you dump him and even then no one will give you a cookie for that.
And if he isn't as attentive as he should be for the pregnant ex/ hookup mama or the baby because he thinks it will make you happier then he is kidding himself. You shouldn't want to be with someone who wouldn't be present 110% when they are having a kid. The child is the most important part and him making sure the mother is healthy and not going through a pregnancy alone is important. ...plus all the baby proofing and life changes he's going to have to make ...all mean DRAMA FOR YOU :s
You already know what you should do. Most people who come on this platform are looking for validation of what they already want to do or for someone to say "everything will be good if you stay because I did it and my life is great". Gigi from "He's Just Not That Into You said it best.. good movie.. watch it!
I don't know how old you are but regardless of age, life is too short to waste it on drama. Breaking up with him is for his own good but more importantly it's in your best interest. You dated for 3 months. That's not that long. You will meet someone else.
If you caught feelings then suck them back in and keep moving. It sounds heartless but ain't nobody got time for that. It's only been 3 months and he's already brought this kind of drama to the table... You don't need that. Dating is for meeting new people, having fun and hoping to find something real with someone...not getting babies...and baby mama's...all with someone you don't really know that well. Babies are cute but they are work.
Keep it moving. You are just making room for the right person to come into your life. I'm sure you're a lovely woman and can spin in a circle and pick out someone more available than this fertile man. When something is meant for you, it won't end up like this. Call it karma, the universe, cosmic intervention or even God.... this is just not for you.
Do what you want to do though. All I'm saying is I prefer my drama on tv than in real life.