If he's been drinking a lot of water, seems thirsty, urinating more frequently, and the urine has a stronger odor than usual, then it may be due to a Urinary Tract Infection (UTI). I've never had one of my dogs do this, but it's what I've heard. The only other thing would be if the dog wasn't getting enough water to drink, and was becoming dehydrated.
It shouldn't be harmful for him to drink it, but if it were me, I'd head to the vet and get him checked out...or at least collect a urine sample, and see if they'll run a test on it without the dog present. Once a medical condition is ruled out, you can just try to discourage the behavior by watching him and being nearby when he goes out, and then correcting him with a 'no' or 'leave it' command. Maybe you can run a bit of water over it with the hose, so he gets the idea, and isn't so tempted.
"We can judge the heart of a man by his treatment of animals." ~Immanual Kant~