If he's not drinking, try ice-cubes in your hand - may help him to get some water. If that doesn't work, and he's not drinking, you might need to try some kind of syringe. Dehydration is common if the dog's been sick, I think.
I'd definitely try to call the emergency vet, or take him in first thing tomorrow.
It may just be a tummy infection, which is very common, but some of them can be very severe and worth the vet checking him over.
Hope he improves soon.
xx