K.G.
It sounds like he's not scared of the potty, which is good. With my three-year-old, I eventually had to just bite the bullet and put him in the thick cotton training pants. If he did have an accident, I wouldn't get mad, but I would tell him that he was too old to be going in his pants and I would stand him in the bathroom for a few minutes before changing him. Again, I WASN'T MAD and didn't punish him ~it wasn't "time out"~ or speak sternly as if he had done something bad. I was just very matter of fact that he was too old for this and that I didn't have time to clean him up just then. I would go do something for only a minute or two, but he didn't like standing in his wet clothes. I would remind him that if he just went in the toilet, he wouldn't have to be in wet clothes.
I only resorted to this after trying everything else, and I still took him to the bathroom often throughout the day and reminded him to go. I just think he didn't mind his diapers. They are so absorbent that they weren't uncomfortable to him, so he wasn't very motivated to go in the toilet (and yes, we also did M&Ms and a sticker chart). Once he was uncomfortable, he was trained in a couple days. I think he only had 3-4 accidents. Good luck... I know it's frustrating!