I've had him for almost 4 years, it's never been an issue. But I've noticed that even after a long hot walk he just isn't interested in water. And I never see him drink anymore. I try to get him to but he just gets irritated with me. It's been going on for well over a few months and he's still, you know, alive... but his urine seems concentrated and I think it's really weird.

He has to be getting some water, somewhere, because dogs aren't made to survive only on the water they get from food, like some other animals. Can he get access to the toilet? This is a favorite 'spring' for many dogs and would satisfy his absolute need for water. As you have noticed, his urine is concentrated, which indicates a need for even more water.

A trip to the vet is the best bet for him, even if you do find where he gets his drinks. That could become problem in itself, depending on where and what it is.