I'm not a fan of Cesar Milan's techniques, especially with a fear aggressive dog. I have seen more issues than results from showing dominance to a dog who has no self confidence. They tend to know who's boss, and unfortunately, that boss has not always been nice, so the techniques tend to backfire.

Now, my disclaimer is that I am not a professional dog trainer, but I have trained search dogs and several of my good friends are aggressive dog trainers, so we talk a lot. I think the most important thing you can do for Rusty is to give him plenty of exercise (a tired dog is a good dog) and plenty of training work in something like basic training (positive only and plenty of rewards) or basic agility. Know that training can have the same affect on a dog as heavy exercise (think about how tired you are after an exam), so start with short 5-10 minute training sessions.

You want to build his confidence and let him know that he can be successful. You said you were working with a trainer, which I think is so key - I'm so glad you're doing it. Ask her what she recommends to work on. It can start as basic as "sit". Use a clicker or another marker (verbal "Attaboy" or something) and treat when you click. Even if he already knows it, it will help him realize that he will get rewarded for specific behaviors. Dogs do what works. It's important to show him positive rewards for the behaviors you wish to see. What you are doing is giving him confidence that certain behaviors are appropriate to you, the pack leader. When he knows what he should do, it's easier for him to know how to behave properly and his confidence grows - the more confidence he has, the less fear, and the less fear, well, you know... (If you get a chance to get a copy of "Culture Clash" by Jean Donaldson, I highly recommend reading it - it's a great book).

And, seriously, I absolutely understand that if he bites your son, there is no compromise. Sometimes that just has to be the answer, unfortunately.