Jaybay is right. Definately get the dog into the vet and get all appropriate tests done.. This happened to my golden at that age 2 years ago and unfortunately our vet at the time wasnt helpful at all and said she was fine when in fact she had a couple of things going on, one being cancer and we had to put her down and it was awful... so dont wait on this and goodluck.
Oh dear. That really concerns me. Any time a senior dog behaves this way, you need to jump on a vet exam. This is the time of life that various organs can begin to lose their function, so he should have a full blood workup to check kidney, pancreas, liver function, etc. You might as well bring in a urine sample as well when you take him in. Hope it turns out to be nothing more than a virus, but don't wait on getting him seen.