I am sorry that you have been waiting for a response, but your requested expert isn't online which delayed your question coming up on the list for all to answer. I would like to help if you are still interested in an opinion.
I am sorry to hear that your fellow is drinking and urinating a significant amount more then usual.
Is he eating less or is he more hungry than usual?
Is he losing weight?
A normal dog will take in 1 to 2 ounces of water per pound of body weight in a 24 hour period. That includes drinking and water in their food. So a 75 pound dog will take in 75 to 150 ounces in a 24 hour day (10 to 20 eight ounce cups of fluids). They drink more when eating dry food and less with canned because canned is primarily water. I would measure the amount of water he drinks in a 24 hour period to see if he truly is drinking too much. Put out a measured amount and then keep track of any added and subtract what is left at the end of a 24 hour period. If he is eating dry food and it is warm where you are and he is drinking more then 2 ounces per pound of body weight over a 24 hour period or more then 1 ounce per pound of body weight if he is eating canned food and the weather has been cool then he should be seen by a veterinarian.
I would be concerned about internal organ disease (especially kidney disease, but liver disease is possible too) or an infection, especially in his urinary tract (if he is not neutered a prostatic infection would be a concern) as well as diabetes or some types of cancer or endocrine diseases that cause increased water consumption and thus increased urination.
If he is eating poorly then his loss of appetite can be due to increased organ waste causing gastrointestinal irritation or by products of an infection suppressing his appetite or a mass taking up abdominal space and putting pressure on his stomach and intestines such that he doesn't want to eat.
If he is eating well but losing weight then diabetes, early kidney disease with protein loss or some endocrine abnormalities may be at fault.
I highly recommend that he see his veterinarian for an examination, complete blood count and biochemistry profile as well as a urinalysis and maybe abdominal radiographs or an ultrasound to diagnose his condition.
If he has pale gums and tongue or a bloated looking abdomen I recommend that he been seen as soon as possible as that could indicate internal bleeding or a large abdominal mass.
In the meantime although it is tough on you please do not restrict his water, let him have as much as he wants as his body needs the additional fluid intake.
Please let me know if you have any further questions.