Well, I was recently the pregnant ex girlfriend, so I can only speak from my experience; and it IS biased. There are many details that I will not go into simply because there are just too many. I can however, touch on some major points. My son is 10 months old now. My ex dumped me the day I told him I was pregnant. He literally pulled the rug out from under me, so that was devastating enough. He moved back in 4 months later under the guise of 'wanting to work things out' and said he didn't know what he was thinking. Meanwhile, 2 months after he left me he met and began dating another woman. Within 2 days of him moving back in I already felt that something was wrong. Turns out he was living with me and leaving every weekend to be with his girlfriend. He wouldn't show me any intimacy at all. Within 2 weeks of this I found texts and pictures and I kicked him out. He constantly denied having a girlfriend and even went as far as telling me she was his cousin. Ultimately I called her and the first thing out of her mouth when I told her who I was, was 'Oh, You're nothing, you're just the baby's mother'. Long story short, I went through my entire pregnancy alone. He would come to appointments and show up once in a while, but I was left taking care of a 10 year old, working full time, and doing all of the other required duties to run a household. At the same time, he continued to deny everything including the fact that he dumped me. I corrected him every time he said 'we broke up'. On my end, it was really a miserable, lonely, difficult torture of a time.
Okay, that was a bit longer than I intended. And yes, long horrible miserable story. In your situation, you don't say who left who, or if it was mutual. And when you say they haven't been together since 'is what you gather', leaves me (and it seems you too) lacking some detail. The first thing that I thought when I found out he was with someone else who knew I was pregnant was 'what kind of dumb b*tch would get with a guy with a pregnant girlfriend? she must live on drama..'. That is by no means a judgement on you, as I was devastated and furious at the time, and as I said, my take on this IS biased..
The important thing is making a personal judgement right now on him. So here are some things to consider. The same time my ex was cheating on me, she did not know that he was living in my house. So #1, he was playing both sides, as she too was in the dark about what he was really doing and you seem unsure as to whether you could be in the same situation. #2, the possibility always exists that your bf will decide to get back together with her. Also, after a few months, my ex dumped her too. So #3, you have to consider the overall character of the man despite what he does. I know that I felt absolutely horrible to find out he was with someone else and it hurt so bad that he showed me no affection at all while I was pregnant. The entire situation is a mess and has too many variables that do not point to an ending where all 3 parties will be happy. In my opinion it takes a 'special' kinda man to leave his pregnant girlfriend. My advice? Leave him. You've only been together for a few weeks, this is a horrible situation to be involved in, and that feeling of jealousy you have when you witness their 'bond'?? If you feel it in your gut, don't ignore it. Lol, flee..
Sorry this is so damn long, but it was pretty recent and I speak first hand. In the end he is always late with the small amount of support he is supposed to pay, and always late to visit his son. Overall, he's just not a great guy. You're just about right on when you spoke of putting yourself in the ex's shoes. None of this is probably worth your trouble at all.
(Hey peeps ^_^ been a while...)
“Be who you are and say what you feel because those who mind don't matter and those who matter don't mind.” ~ Dr. Seuss