Kids are so different and yes, PT can become a power struggle. With my daughter, all it took was a reward system to get her started at 2.5 and she was on her way. We had a couple of set backs, but they were breif and relatively minor.
My son, on the other hand, is much less interested in the whole process and we have had several false starts already. Like you, we've had several sessions where he'd "try" for 20 minutes only to have an accident 5 minutes after leaving the bathroom. *sigh* I don't think that it's deliberate but he just can't be bothered to pay attention.
As much as he'd like to wear his big boy underwear, I've made it a priveledge to be earned and he understands that. If he uses the potty every time for a whole day then the next day he can wear underwear. More than one accident in a day and it's back to the pull-ups.
I try to stay unemotional about accidents and even about the days he has more wet pull-ups than dry ones. Successes are celebrated, of course, and ever-so-slowly we're making progress. It's taken some very different incentives, though! We celebrate "dry flags" on the pull-ups at regular checks and use it as an opportunity to "try" on the potty. We've tried sitting on the potty and standing up, aiming for cheerios in the bowl, and making "letters" in the little potty. We've called the grandparents on the phone to celebrate a poop in the potty, although 90% of the time he'll still hide somewhere quiet and poop in his pull-up.
Kids are just ready at different ages and they require different motivation. Many moms have told me that their boys took longer to PT than their girls. Maybe it's a kind of emotional maturity. Maybe it's a higher threshhold for messiness or discomfort. Maybe it's just that they can't be bothered to interrupt their play! Whatever the reason, some kids require more time and patience.
If I were you I would explain that you're going back to pull ups until you have a "dry day". He may protest, but that's OK. You can explain that he's still potty training, but that he needs to be able to go in the potty every time for one whole day before he wears the "big boys" again. Make this very matter-of-fact, not as a punnishment but just the way it is and that it's ok. It can be the first thing he works towards.
Then I would go back to trying at regular times during the day. Try not to get upset if he's wet in order to avoid this being about power and control.
He will make it! You're doing all the right things and kids are just ready in their own time. It may take him a while longer but he *will* get the hang of it! Good luck!