Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I understand that you are concerned because your fellow Rambo is drinking more water than usual.
Most normal dogs take in between 1 to 2 ounces of fluid per pound of body weight per day. True polydipsia (increased water consumption) is when a dog is drinking more then 2 ounces per pound of body weight in a 24 hour period without other demands (such as very hot temperatures or excess exercise). So for a normal 50 pound dog he should be drinking 50 to 100 ounces per day. That's 6 to 12 cups of water per day for a 50 pound dog.
Things that can drive an increase in water consumption and subsequent increased urination range from a urinary tract infection, urinary calculi (stones in his urinary tract), to a prostatic infection, to developing kidney or liver disease or even diabetes.
Less commonly we see increased water consumption and urination with diseases such as Cushing's disease, or even some types of cancer (lymphoma or anal gland tumors), diabetes insipidus, or a decreased production of aldosterone, which is a hormone that stimulates the kidneys to concentrate the urine.
The most common cause of increased water consumption and urination in dogs is a urinary tract infection, and with a mild or early infection you may not necessarily see any blood or a change in the way the urine looks, and in older dogs early organ failure.
If he is drinking a noticeably increased amount of water compared to usual I would have him examined by his veterinarian and check a urinalysis to start. That test isn't very expensive and will tell us if he is spilling protein or glucose into his urine, which could signify kidney disease or diabetes, and can also allow us to look for signs of infection.
Depending upon what we see on his urinalysis then some blood tests may be recommended too. There are usually mini geriatric profiles available that may not be quite as extensive, but will be more economical and may give enough information.
In the meantime I do not recommend limiting his water, in most cases that could lead to dangerous dehydration.
Please let me know if you have any further questions.