T. -
I have a 3 1/2 yr old boy who is finally potty trained for peeing but won't sit on the potty for pooping ever - he is afraid of sitting for whatever reason. he knows to ask for a pull up to poop and knows to ask if its ok to poop in his pants ( i say gently, yes, you have a diaper on now and its ok to poop, I'd be really happy if you'd try to sit on the potty, but if you'd rather poop in your diaper that's ok - I know you will try the potty when you are ready. I also thank him for asking if it is ok to poop - which lets him know I really appreciate his asking me...)
He now can hold it all day and usually just asks for pooping diaper right after bath and before bed - so accidents are not likely anymore - and I can live with one diaper a day at night...
I used this same philosphy and words with the peeing part, too - as he was also afraid to pee into the potty for the longest time - and finally he decided he was ready...
for him it is both a fear and a control thing - both normal and no reason to rush him...he will go when's he ready and the good news is that he won't be in diapers when he's in high school or getting married, as they say...
good luck to you - I can totally relate to how frustrating it can be but just remember he's not trying to do it to make you crazy on purpose - there is an underlying reason and if you ask him what it his - he will probably tell you like mine did - then you can go from there using the right words to reassure him...