Leave him alone. Let it go, put him back into pullups and let it go. Figure out something that he wants, his currency, does he like Transformers, legos, super heroes?
Make him earn it!
Get a poster chart, put up a happy face for every time he tries, two for every time he goes.
I don't believe at all in putting kids in underwear until they are 100% done, it is a mess, unsanitary and causes us moms way more stress then we need.
I say, tell him he is a not a big boy yet and when he is ready he needs to put on his underwear. Period.
Be matter of fact, take a few big boy priviledges away and just don't say a word about it.
PUT him on the potty every hour to hour and a half, don't ask do tell just do it. Be nice about it and just say happily "we are going to try and if you try you get a smiley" tell him if he gets 40 smileys or something he can get a little surprise. Reward him once he is in underwear with whatever toy he has really been wanting (within reason).
You won't win this power struggle, take off the underwear until he is ready mentally too along with all the other signs of readiness. All it took for my son at 3 was wanting a Transformer which I told him was a big boy toy, for my daughter it was Santa calling her before Christmas, so all kids have currency that will help get them over the stubborn hurdle.