Where is he shaking? His body? Or his face? When their faces shake, it's because they are bruxing..It is when they rub their teeth together and their lower jawbone presses on the backs of their eyes, making them pop in and out, and making their faces jiggle. But if that's not it,and it's his body, then I'm not sure what it means I'm going to the vet today, I'll ask for you