Myles, I'm trying to think of some good advice for you and I have some conflicting ideas in my head.
Part of my wants to say that it is best to remain calm and unemotional.
Another part of me wants to say that with some of the kids I worked with, we made a point of saying - dramatically enough to get the point across but without anger or hostility - "That hurts me." And then move on/ignore. If he is seeking attention or a reaction my second suggestion would be counter-productive, but if it's simply a reaction in the moment to being removed from the situation I suspect increasing his awareness that you are being hurt is not a bad thing.
I absolutely do not think it is coming from anything he has witnessed in you. I unfortunately have seen a lot of kids with severe delays and it seems that the lower the skills, the more "violent" reactions can get and often there is no violence on the part of their parents or caregivers. I believe those physical reactions (hitting/violence) come through when kids are just so far out of themselves that words fail them and they resort to pure emotion. So don't think of it as something you did or he witnessed, think of it as how much his emotions are affecting him/making him forget all he's learned in how to interact with people in a rational/social way.
I definitely agree that it's best to prepare in advance to avoid situations like this. Really when you're in that moment of hiting/biting you've reached crisis level, and I was taught simply to not allow yourself to get to crisis management. Not to say I haven't been there plenty of times, it's just that when you're there, there's not a lot that is helpful to get out of it. You just have to let it pass. Sometimes that means hands off and wait it out, like Chrissy described.
Hope it passes quickly. It is a phase and a normal one for his age.