I wonder why are you so angry? Why are you projecting your anger towards your father on him?
I think you need to step back and look within as to what the root of the real problem is, and not be so hard on your husband. He probably simply isn't as outspoken as you are, and you need to let him deal with things as he sees fit. Like someone else said, if you yell at him and say these things in front of your son, why would he treat him any differently than you are?

I know it's frustrating not have him stand up for himself, but to demean him for not doing so just makes him withdraw more and makes him feel like crap. No one gets encouragement from someone who yells at them, no matter what the reason.

I think you need to calm down and reread what you wrote. Would you like someone saying such things to you? I know it's easier said than done, but try not to react to him with such animosity. By doing this, you are reinforcing your son's behavior and letting him know that it IS ok for him to demean his father.

I was irrationally angry myself last week, and was thinking of leaving. But stepping back I realized that I was just being a jerk myself, and apologized profusely.

You gotta think about how you would feel when you talk to others like that. How would you feel if the roles were reversed?
I hope you and your husband work things out. I think a simple talk between the two of you would help. It's funny how wrapped we get in everything else and forget about those we live with. I've been guilty of the same.
Take care.