Are you planning on going on with the pregnancy?
If you are, his commitment issue may step from that- not wanting to be a a dad of someone else's child.
If you are not going on with the pregnancy, it may be other things causing his lack of commitment. You've shared that during the 3 years of dating it was on and off. He may not be a men who's into commitment or long term serious relationships.
You've already told him where you stand. If he had ignored your wishes, then there is not much you can do or say to influence him to change. Find out what does he want and what he's willing to do. Is he willing to compromise, take some time to see how things go on b/w the two of you, or if he just wants to "hang out" with you. The fact that he's seeing someone else while he's also hanging out with you and is having an intimate relationship is a red flag for both you and the person he's seeing.
There must of been issues that caused the break up. If they were not worked on and are not being addressed now, it would be difficult to reunite with him. It is hard to move on and you do not have to forget him all together. You would just have to do what works for you. It seems that he is seeing someone else and you and yet is telling you he loves you. Even if he is confused about what he wants, he is giving you mixed messages and that is not healthy. He cannot commit, he is dating someone else and you on the side it does not seem a healthy situation to want to be in. Perhaps all the factors that had led to the initial break up would help you view the situation objectively.