Quoting Brandi Rodrick:" Just curious here.. why don't you want him around his son?"
Did you not read the part where he threatened her life? I wouldn't want someone like that around my son either. What if he gets mad at the baby?
OP, he can't legally take your son until the court orders it. I would be recording hostile conversations between the two of you. Go to a lawyer asap and file for sole custody.
Why do you have an open case with CPS? Unless there is proof that your son is living in a hostile environment, I don't see why they would be taking him.