Pregnant with twins and ready to call it quits!!!?

My ex-boyfriend and I have a 5 year old son, and have twin boys on the way. My ex hadn't been a part of our sons life much, and had little to no financial responsibility for him either. About two years ago he filed for visitations, which was strange because I never told him he couldn't see the baby, but I did let him know that he needed to be financially responsible as well as be there for the baby. I guess he thought that it would be easier for him to file court papers and go to court. I didn't fuzz or anything because I figured finally he wants to be a good father to our son. We went to mediation where we settled visitation, and later court decided on child support 250 per month. Well little Mr. never picked up our son or paid child support. I was mad because honestly it was a pain in the *** going back and forth to court. The case was a year and a half went by, and by this time they had suspended his driving privileges. I had really put everything behind me because well my son knew very little of him and was more attached and still is more close to my father. I got a summons to appear in court because of the child support issue. My ex was about to go to jail because he owed me more than 4,000 dollars in child support. I was really just fed up with interviews and court and all sorts of questions about him. At this time the child support office had the case and was the one filing everything for court. He asked me if we could settle it privately and I agreed, I was tired of court and just wanted it all to just go away. In the beginning he did start paying the child support to me but he stopped a couple of months later. I didn't get mad or anything I knew it was kinda too good to be true. He later came and told me that he had gotten a divorce from his then wife, apparently she asks for allot of money for their two kids. I felt bad for him, i was just really ready to put it all behind me and get on with my life. So time went by and he text-ed more often usually asked about our son, I was like ok? Then he asked about me. I figured that to him I had become that shoulder to cry on I tried to convince him to go back to his ex and try to work things out with her. He refused pride or something. The point is before I knew it I was his best friend and I was stuck in a relationship that I did and didn't want to be a part of. In part it was great because we hanged with our son. But it was also weird because I wasn't sure how long it would last. I started to feel myself fall for him again, so asked him for space. But it didn't last long because he'd surprise me at work and at home. I guess I allowed myself to fall for him despite everything. We began dating and messing around. Well needless to say we didn't use protection, and now we're pregnant again. In the beginning he wanted me to have an abortion. I told him that if he didn't want to be a part of our new baby's life it wouldn't be a big deal for me considering that he wasn't there for our first son. After a while he started coming around to the idea of being a father for the fourth time. And he moved in with me. On our 20 week ultrasound we found out we were having twin boys, He again kinda pulled away. We're 27 weeks pregnant now and he still can't deal with the idea of us having twins. I have bought everything for the twins. The only thing that he's done for them was paint their room. But i had to practically beg him to do it. I'm honestly getting tired I realize that our relationship moved along kinda quick but honestly you think he'd get with the program. I just don't think we're going to make each other happy. The question is I honestly think that my sons should carry my last name. I feel really confused because for one i would like my sons to know that yes they are all from the same dad. But in the other hand i don't feel like he deserves that privilege to call himself my sons father. Oh in case your wondering he's been cheating on me with internet girls and his ex so no there is no way we're getting back together.