Am I correct in understanding that you're taking him to the potty every 15 minutes, whether he needs to go or not?
Honestly, that could be your problem right there.
You're absolutely right to have him in big-boy undies, but if you're taking him to the potty all the time, like it or not, he doesn't have to pay attention to his body's own signals. Potty training is ultimately a mental process -- the brain needs to recognize the body's signals and connect that to an action. You're doing his thinking for him!
What I recommend instead is: let him run around naked. If you have hardwood or tile floors, he can do so inside. With carpets, you may have to limit that to outside. Boys, much more than girls (and men more than women) tend to be visual thinkers. They have to SEE the pee streaming out to make the connection of "Oh, that feeling leads to this pee, and this pee belongs in the potty." It's also worth having a potty in whatever room he's in -- so he can make a visual map of pee --> potty. Then, once you get some traction there, have him in just underwear, no pants yet. But, let him have accidents! That's part of the learning process.
Oh, and rewards! I've always limited sugar with my son, but during the potty process, I got so desperate that I started giving him 1 skittle per "success." It worked, and it was so successful that I was able to phase out the skittles within a few weeks.