What would you regret more? NOT having another child with this guy, or going ahead with it, then ending up a single mother again, but this time with 2 mouths to feed? Bearing in mind that vasectomy reversals don't always work, and adoption is always on option too.
Being with someone for a year is not enough time to be asking him to reverse his vasectomy and consider having more children with someone that he is not married to. He is already paying child support for 3 kids from 2 other women - is he supporting you and your child as well? Sorry, I know I will probably catch a lot of flack for this, but wanting to have a child with him at this time is foolish. Your boyfriend is being smart about this - if he did have a child with you, and things did not work out, that would be a 4th child he would have to pay child support for (or is your child his as well? Because then that would be 5!).
Give it time. If you guys stay together and decide to make a firmer commitment and get married, and you can honestly afford it, then maybe you can talk about having more kids. But if he is adamant that he is done having kids, respect his wishes - then you need to decide if you still want to stay in this relationship, or move on.