So sorry to hear this. My pit, Marci, is a rescue and she's got some fear issues outside but it manifests differently. I also understand what you're saying. I've always told myself the same thing in regards to a biting dog. Obviously it's much easier said than done.
It sounds like A.) you've got a serious decision to make. Are you willing to spend the time and energy at rehabilitating him? B.) If so, you're going to have to turn yourself into dog behaviorist/therapist. C.) Since this behavior is new, you have to take a HARD look at your leadership. You will have to be positive that you're honestly and openly looking at your behavior, and if there are any possible ways it might lead to insecurity or uncertainty of his place in the pack.
BTW, what has been your reaction to his snapping? Are you immediately flipping/submitting him? Will he let you? Are concerned about him biting you in this situation? If he's not stable enough for you to take full physical control of him, even in fear state, my hopes wouldn't be high. If he's stable enough to recognize your leadership/dominance in fear state, my hopes are higher.
My heart really goes out to you Jen. I'll keep you and Louie in my prayers.