that would concern me. I'd check his gums to make sure they look normal not pale or cold. If they are, i'd prob go to an er vet. if they look ok and he has no other symptoms, put a blanket on him and you might see how he is tomorrow.
eta: I'd listen to my gut on it. But, my dogs are very very healthy so any little thing where I'm worried and I know its probably NOT a 'little' thing.
I hope its nothing!