Potty Training

Ughhhhhhhhhh.........I HATE potty training! We have no idea what we're doing.  We've tried everything and we still have a very stubborn 4 year old running around in diapers.  Again, I find myself wondering if it's his stubbornness or his autism or a combination of both, but whatever it is I'm ready for it to end.  He's ready.  He'll tell us when he's dirty.  He can take down his pants and pull off his diaper.  He will stop what he's doing and focus on urinating.  He's ready!  We've read books.  We've scoured the internet looking for ideas.  We've offered candy.  We've had him run around naked.  We've made visual cue cards.  I don't know what else to do.  He'll sit on the potty for several minutes and nothing.  And then as soon as the diaper is back on he'll fill it to the brim.  In all of our attempts, we have NEVER gotten him to urinate while sitting on the potty.  Thinking that maybe the problem was sitting on the potty, we've also tried the standing up approach.  This hasn't worked either.  We are at a loss.  Anybody have suggestions?  If we don't have some sort of breakthrough in the next few weeks, our son is going to be running around naked all summer while we try to get him used to life without diapers.  Neighbors - consider this your warning!!