ok.
what ive learned from being a child care provider to a child who, just like your son, had NO interest whatsoever in being potty trained. i think he was ...... just over 4 when he finally 'got it'
first of all, NO PRESSURE. dont push, dont plead, dont offer rewards, dont offer punishments. NEVER EVER PUNISH for a potty accident.
going potty is a very personal thing. very. only he can decide to go or hold it, theres nothing you can do to make him go... nothing at all. the more you pressure him, the more resistence hes going to show. if its something fun, relaxed, that HE can choose to do on his own in his own time, then he should do great all on his own.
i tried everything with this boy, and mom did too. mom flat out asking him "do you like sitting in your poop?" only received a "yes"
so, have the potty accessible. leave books or some kind of stationary toy there to distract and keep him company. dont sit there and hover over him. (can YOU poop when watched? i know i cant. i have enough trouble in a public restroom when someone else is in another stall!)
periodically check on him and ask him how its going.
one thing physically, there are only certain windows when someone can poop. when you feel like you have to push the poop, thats the time. if that time passes, or isnt yet, there is NO way he will be able to poop.
what you can try is in the morning, on top of cereal or whatever, sprinkle some milled flax, and make sure that he gets plenty of water. the flax has fiber, which absorbs water, making poop softer and easier to pass. i tried this with a baby i had, and i swear, it was within a half hour he had to poop. i didnt know about it until after the 4 year old was trained, so i cant say how it works on him... but try it. and watch, in about a half an hour have him sit on the potty and read or something. if he gets interested in movies easily, and will sit for them, thats a great idea too.
good luck mom. potty training is the hardest thing - and again, only he can choose to be trained. most of all, have patience, relax, dont push or pressure. no reward no punishment. he is also old enough to help clean himself (and mess) up. so try that too.