Sorry you're going through this.
I'm not surprised your son is not angry, I supose by now he knows what he can expect from his dad. And aslo in my experience when kids face things that are overwhelming they don't know how to deal with it so they tend to get a bit numb and pretend nothing is happening. IMHO that is more damaging to them than when they act out. If I were you I'd get my son some counseling, it would be good for him to have someone he could speak to about it. I know you speak to him but that is not the same thing as he can not tell you how he really feels, as it is most likely, seeing you go through all the problems with you exAH your son feel protective of you.
As for the mother of the boy, it sounds to me that she said that only trying to make you feel better about the whole thing. At least that is what I'm reading from it.
On the more practical side I don't know what exactly you can do, is there any chance you can use this DUI for court order that would mean your son is not to spend time with his dad alone any more or something along those lines?
I have you in my prayers.
HUGS