My question involves paternity law for the State of: Florida

My ex and I were together at the time I had my son. He signed the birth certificate thinking he was the father. We have not done the DNA test, but we are 200% sure he is not father, my son looks nothing like him and the timing was totally off. Anyways, my son is 5 now, and I've been trying to get his name change for years but everyone tells me different approaches. How do I go about removing my ex's name from the birth certificate and removing the last name from my son? I do not know where his real father is. PLEASE HELP!