Suing for child support?

I am seventeen and have a three year old son. I was with his father for five years until my son was born. The father was illegal the whole time. I left his father when my son was a year old and haven't heard from him sense. My son is almost four now and i am receiving ABC vouchers and DSS wants me to sue the father for child support. The father is married now and legal. I do not want his father to have anything to do with him at all. I do not want my son to have any communications with the father what so ever. How can i make sure that if i do pursue child support that he will not get partial custody or anything like that. How can i not sue for child support and still receive ABC vouchers? What on earth can i do to make sure that my son has nothing to do with the father?