I agree with serenity. One day, when you think your child is old enough to understand, you need to speak with him about his father. He carries the addiction gene too and he has a good chance of being an addict, even without his father in his life. I imagine that your child will understand that you did not allow him to have a relationship with his father because of his dangerous lifestyle and he will be thankful. Also, unless you just don't want to get into the mess, it may be possible for you to get child support from him even without visitation rights. But in my opinion, your childs safety and well-being is more important than anything else.
And yours too.