Establishing Paternity and Visitation Rights

I do not know what to do. I have a 6 year old son out there. I dont know any contact information for him. I know the city he lives in but no address or phone number. His mother makes sure I dont know where they are. The last time I spoke with her she said that he was not allowed to know I was his father. So now I wonder is he mine? WHy wont she let me see him or know where they live. I am supposed to have visitation but I have not seen him since he was 8 months old. I want a DNA test done and visitation rights. I DO pay child support and my tax check gets taken from me every year. That is not fair to my wife and the 3 kids we have together.