When a horse is actively aggressive with a handler, it is a sign of blatant disrespect...does it stem from his lack of confidence in people in general? Probably, but I think where your mom needs to start is ground exercises...getting him to move his feet in all directions; forwards, backwards, left, right. If she can't control his feet from the ground, then of course he is going to be even less responsive from the saddle.
As a whole, I think both of you need to work on desensitization exercises (something I think is very important in abuse cases, and otherwise spooky/flighty horses). Use flags, plastic bags on sticks, etc. I find that the more desensitizing you can accomplish on the ground, and the more engaged you can keep his mind both from the ground and undersaddle, the less flighty they tend to be.
My mare was very spooky when I got her too, and I've done ALOT of desensitization on her, and that has helped immensely.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."