I had a cat with chronic constipation and understand how exciting it is when they do have a bowel movement!
One thing I would urge you to do is to switch to entirely canned food or greatly increased the canned food and decrease the dry food. The dry food causes a kitty's body to use more water to pass the food. If your kitty is already not drinking enough water, this just compounds the problem. Canned food has a lot of water content and helps bowels move easier.
You actually could measure the water. Measure the amount of water you put in each bowl and 24 hours later you measure what's left. That way you'll know how much he drinks in a 24 hour period.
Cats really shouldn't drink that much though. In the wild cats get most liquid from their diet. Cats that eat dry food should drink more though and of course lack of fluids will cause constipation. You may want to give him some canned food in gravy to get him more fluids. Another thing that usually works pretty well is tuna water. I have yet to meet a cat that doesn't love the stuff.