I have a two year old who does the same thing when he's in training pants (disposable are a waste) or underwear and also does well when he's running around bare bottomed. I don't know if your son has expressed an interest in going to the bathroom or if this is something you're pushing but do realize that THEY, not us, control when they'll be potty trained. It's one of the few things that children this age can control. So if it's becoming a battle, prepare to lose.
With that said, if he's ready, I'd let him run around commando. Let him pee in the grass outside for extra fun. Put Cheerios in the toilet and let him aim at them. If he doesn't get it, no biggie. Just continue what you're doing or go back to diapers. If you go back to diapers, use a good quality one and change him frequently to help cut back on diaper rashes. Also make sure the doctor's taken a look at it. My son had a horrible diaper rash that turned out it was a yeast infection on his poor little bum.
I really wouldn't sweat it. Boys are often late bloomers when it comes to potty training. My older two were both around three and a half when they potty trained and had showed NO interest before then. I'm keeping my fingers crossed that my two year old will stay interested but right now we're back in diapers because it's more an occasional game he enjoys now and then than "hey I'm ready to ditch the diapers, Mom." I'm ok with that, even though the little boy next door (only one month older) has already mastered it all. I'm not Super Mommy and I don't expect my son to be a superstar either. It'll happen long before I need to send him off to school and that's what really matters.