3 1/2 year old will NOT potty train

I have a 3 1/2 year old son that I have been trying to potty train for over 2 years now. I have given him breaks between potty training. My problem is that now he goes into a crying and screaming fit where he will even hit when he is ask to wear underwear or use the potty. He says he wants his pullups. I have told him that there are no pullups and tried to put underwear on him, but he threw a huge fit and sat there bottomless crying, so I gave up. About 3 weeks a go I managed to get the underwear on him, but he did not go pee or have bowel movement until I took the underwear off and put on pull ups for bed (9 hours). He is very smart. He can ride a bike with no training wheels, knows every color created, every shape, can count to 27 and idenify the numbers, know all the letter of the alphabet and can even read small words. PLEASE HELP!!!  His peditrician doesn't even know what to do!

[specialmom]

Sounds like my very bright second child.  He had an aversion, so it would seem, to potty training.  For him, he wanted to be the "baby" and babies where diapers.  He is a smart little guy like your son and has the advantage of a slightly older brother, so he did every thing sooner than most kids.  Everything . . . except potty training.  

I decided the best course of action was to be relaxed about it.  What good does it do to get all stressed out and exasperated about it?  None and I think it actually slows the process down.  I also think . . . forgive me for saying this . . . but you started WAY too early.  The average-----  and this is a statistical average in the US -----  age for a boy to potty train is 3.5.  So he isn't way far behind or anything . . . he isn't in kindergarten in a pull up.  So take a breath and keep this in perspective.  

My smart little one held out until he was a little past 4.  Can you believe it?  We tried various things and he just wasn't interested.  So, I said to him  "son,  it is time.  You'll always be my baby, but It is time to let go of the diapers."  He said-----  okay.  That day, right then and there----  he was 100% potty trained.  He went into underwear at all times and doing his business in the potty with no issue.  He even went straight to standing and peeing.  So I think what you'll find is if you back off for a bit and then talk to him about it without being all stressed and upset . . . you will find he will just all of a sudden be potty trained.

ps,  my boy who was potty trained a lot younger than him took like 4 months to get it right . . . late has its advantages.  good luck