How do I prove paternity?

(Ft. Lauderdale)

I'm trying to get visitation with my son. I lived with his mom, but we were never married. After we split up, she was fine for a long time, and let me see him just about every time I asked. Now she has a new boyfriend, and they're not letting me see my son at all. My boy is two years old now, and he definitely is mine. Everyone knows that he is my son, he looks just like me. My name is on the birth certificate. What do I do so they can't keep my son from me anymore? Any help is greatly appreciated.