Well, I think, that the Lack of flight-capabilities is compensated by superior strength. And when it comes to reaching a place really fast, Spike might be able to use his magical fire to teleport himself. At least those two are my theories about him.
Given the size and mental state of the dragon assaulting said jaw, it could be made of diamond and he'd still be knocked flat. We are witnessing the majority of his physical sensation for the better part of the next week.
Give that dragons eat diamonds I don't think breaking them would be very hard for them. Also i was making a joke about a crystal jaw because their in the Crystal Empire. But saying he broke diamonde does sound cooler.