Did you get him from a reputable breeder? Sometimes when dogs come from puppy mills, they have no choice but to soil in their sleeping areas... nobody lets them out. While they have the instinct (I assume) not to, they probably can't hold it very long.
I think the key to potty training is prevention. I know you said you've been taking him out frequently, keep doing that. Also, while he is out of his crate.. WATCH HIM LIKE A HAWK! The minute you see a sniff or circle, scoop him up and take him outside. I know it can be very frustrating but I think patience and persistence is the key.
I would also put him on a feeding schedule if he isn't already. Potty first thing in the morning. Then feed breakfast. I've heard that dogs usually have to go about 15-20 minutes right after they eat although in my experience, it's closer to 45 minutes to an hour. Take him out after naps, heavy drinking, and heavy playtime. And then every hour or so like you are currently doing. I would also limit food and water a couple of hours before bed so that he doesn't have to go throughout the night.
Lastly, is his bedding fabric? I've heard that when crate training a puppy, you shouldn't use any blankets or towels because they are more prone to pee on absorbent surfaces. Also, if peeing and pooping in his crate was more uncomfortable for him, he might do it less.
Just be patient and keep at it. One day (even by accident), he'll go outside and when he does... go CRAZY!
Praise and treat like never before and eventually he'll get the idea.
Good luck.