Soon to be 4 Year with Toilet Training Issues

Our son will be 4 at the end of May.  He is the older sibling to twin sisters who will turn 1 this month.  Their birth and homecoming was pretty stressful and after they did come home from the hospital (after three months), about 2 months in - our son was shocked to discover that they actually were going to live with us!  Up until that time, we had been doing a bit of toilet training with him (he was already three) and he was having a lot of success.  By the middle of last summer, he was entirely urine trained and although he struggled with bowel movements on the toilet - he was having a lot of success at home but not so much in public.  After his discovery that the twins would not be returning "home" to hospital - he regressed in almost every way - including toilet training.  We have worked through a lot of the issues and after a bit of a struggle - he has been urine trained since last October.  However, the bowel movement training has been everywhere.  He will go weeks and everything is going great and then he goes weeks and refuses, fights, or just ignores going to the toilet to poop.  This is a big issue with preschool.  He cannot get to the washroom on time or goes in his pants and then asks to go to the washroom.  They have been great with him at the preschool trying to help us all through this process but now it is just getting ridiculous.  I have also learned that other kids call him Pooey Boy.  We have tried rewards, taking away toys and "distractors" such as the TV when there are accidents and even removing him from an activity if he has an accident.  Nothing seems to click with him.  A psychologist recommended that we put him in a pull-up when he has an accident and then when he has a "clean" day - he goes back into underwear.  [We had abandoned pull-ups because they were a cruch for him.]  We tried that for three days and he didn't even try to go to the washroom - because as he carefully explained to me - poop AND pee can go into a pull-up.  He certainly is smarter than we are!  

I am at my wits' end.  We want him to be trained because he is going to be 4 and then he will really start missing out on things (such as skating lessons, hockey, baseball, day camp, etc - all things that he needs to be fully toilet trained for).  He doesn't really "get" the idea that he will be missing out on these things either.

He waits until the last minute to get to the bathroom (I wash a lot of undies) and he hates waiting and sitting there.  I thought this might be good information, too.  Books and entertainment in the washroom don't do it for him either.

Next week is his spring break.  Any ideas on how to approach this?  We are home all week.  I no longer fight or argue with him when he refuses to "try" on the toilet - the battles are a waste of energy.  I have also learned you can lead a kid to the toilet but you can't make him poop . . .

I also wonder if I should pull him out of preschool . . .make him miss his beloved school for a while until he gets it together.  

I am just so lost.  

Any ideas you have would be appreciated.