So sorry to hear you dog isn't feeling well. Glad to hear he is still drinking. This could be any number of things. Is he still not eating after two days? Especially, due to his age, I would be taking him to the Vet for an evaluation. Older dogs can have any number of things going on that could cause these symptoms (kidney, liver, pancreas problems). Some, if caught in time, can be managed with diet and/or meds.
Please keep us posted.
Mine had the same symptoms and the vet said almost guaranteed it was kidneys because of her age. Dogs kidneys go with time. We had her put down because she was in pain after a while spitting blood. I wish we had more experience because the next time it happens to one of our dogs, we will have it put down quickly.
Don't delay in seeking attention as he may last a while for a while, for no good reason.