Thanks for the additional information. I really appreciate it.
Actually, in older dogs, the first thing I think of when I hear that they're drinking a lot of water is that they could be diabetic. Polyuria (increased urination) and polydipsia (increased drinking) are two of the hallmark signs for diabetes.
The symptoms you describe: polydipsia (excessive water drinking) of course can be due to a number of different causes. The most common causes of PU/PD in cats include kidney disease, Cushing's disease (excessive blood cortisol levels), and diabetes mellitus (excessive blood glucose levels). In an 10 year old dog, any one of these disease processes could be occurring.
Getting him to your vet for a thorough physical examination and blood work is the best thing you can do. Try to collect a urine sample as well so that your vet can evaluate kidney function and look for signs of urinary tract infection. Since your dog is a male, you can wait until he starts urinating and then slide a shallow bowl or ladle under him to collect the urine. Put the urine in a clean, dry container and refrigerate until your vet appointment. It's best if the sample is less than 24 hours old.
Other possible causes for PU/PD include inflammation of the prostate (rare in a neutered male), liver problems, or diabetes insipidus (a disorder of water metabolism in which part of the brain does not secrete the hormone which tells the kidneys to re-absorb water, or the kidneys do not respond to the hormone). In addition, there is a condition called psychogenic polydipsia which means the excessive water drinking is all in his head. If he exhibits other symptoms like vomiting, diarrhea or lethargy, other causes should also be investigated. Blood work often helps narrow the field.
I hope this helps!!