Single now, and pregnant

I had a relationship for a year, which was on and off. This past June, I found out I was pregnant. Since then, I'd started noticing that my partner didn't seem very interested in having a baby. I have 3 children already, he has 1. I've known him for a long time, and one of the qualities that I found attractive in him, was how great of a daddy he is to his son. Anyway, long story short, I gave him an ultimatum basically. Get real with me, and let's move in together and have this baby, or get out. He chose the latter.  

Does anyone have anything to offer as far as ideas about how to obtain a child support order? He makes decent money and has had the same job for years. I make less than him, and support my 3 on my income. I have Medicaid for pregnancy, and it is my understanding that any time the mother uses any type of state assistance, the dept of health and welfare will automatically open up a case on her behalf. Is that true, does anyone know? I have an attorney that I used in my last family law dispute, and I can use him again if need be. It's just so expensive. Any information would be appreciated! Thanks!