OP ~ If his girlfriend tracked you down, why didn't you ask her some information on where he is? Did she say when her due date was? You could google birth announcements from papers around that state around the time she is due. I am sure she will list the proud father in the announcement. It may take a lot of work on your part, but if you are really wanting to put the time in, I bet you can find him.
Personally, I had an ex who paid one support payment in 9 years, and that was only because he went to work for a family member. I turned him in and they sent a notice of garnishment. He quit after that. When my new husband wanted to adopt my children, we had to contact the state and it had been so long since any activity was done on that account that it was in archives. They never did find the information I needed, but fortunately the adoption went through without it. I made it without the child support, and it was very difficult. Sometimes I only had $25 a week left for groceries for me and the 3 kids. It built character in me and looking back at the experience now, I believe it was worth it. It made me realize that I was a tough cookie. I just never felt it was worth it to pursue him for the support. It was easier on me mentally, physically, emotionally to go it on my own. But every person is different and if you want to invest that time then more power to you and I hope you get what you are looking for.