Need Some Advice

I am a 21 year old single mom of a 3 year old boy.His dad got involved in drugs while I was pregnant and I broke up with him, long story short, the last time my son saw his dad was when he was three months old and his dad had him laying on the floor on a blanket while he was on the couch smoking crack with his son. When I found out, I took my son away from his dad til he could get clean. Three years later, we are still waiting. Now he is saying he wants to get visitation! He has never paid a dime for child support, doesnt even know what my son looks like, cant remember his birthday and half the time cant remember his name. He couldnt even stop partying to come sign his name on the birth certificate. How do I keep him from getting visitation with my son? I know that he is not clean now and he has also threatened that he will take my son and I will never see him again. Except for running and never letting him find me...What can I do. Some one please help. What do I tell my kid? He is starting to ask about where "his daddy" is when other kids' daddies are around. Someone please help if you can.