I second Red....she will know soon enough who your ex is-they can't hide it forever. You're not being overprotective-you are a good mom and looking out for your child. Keep on truckin'!
And no-of course he hasn't told her why he can't see his own child-that would require telling the truth. You're not a vindictive ex-you're telling the truth-that's not vindictive, that's the right thing to do to keep your child safe.
I'm sorry you are dealing with this, friend. Sorry your son is too. His dad is a dangerous person.