My son turned 3 in January and just this week crossed the milestone of being officially potty trained.
I started letting him run around naked. Granted, he never peed or pooped on the floor, so that technique may not work for everyone.
Once he was able to feel when he needed to use the potty and get there on time, we went to undies. Thing is, briefs didn't work for us b/c they must've felt like diapers to him, and he'd just go in them. So, we bought boxers. That's what worked for us.
We did sticker charts to earn for him something that he'd really really really been wanting, which was one of the Cars cakes you see at the grocery store. We downloaded the sticker chart with a 5x5 grid and put a Cars sticker in each box everytime he pooped (he was already trained for pee; we were just working on the poop). So, maybe find something that your son is dying to have and make him work for it. Maybe, by not switching from diapers, he's trying to get that one-on-one time with you that a diaper change gives him. So, maybe his reward can be going to the zoo with you or something like that.
Good luck. It's a long road, and I feel your pain!