First and foremost: relax! As one pediatrician advises: children seldom graduate high school still wearing diapers... Seriously - being strong-willed and opinionated will be very good for him, when he is deciding the "right" thing to do when ALL his friends are busily jumping off cliffs... however - for now - (and - pretty much throughout his childhood) it will be a "battle" for you, unless you learn to - as they say - "pick your battles"... WHY does he have to potty train now? (I am a gramma - and - believe me - I KNOW it is a royal (smelly) pain - but - what is the issue?) He probably is enjoying hanging at home with M., too! And - it will soon be over, no?
Let's see - generally there was SOME motivator that the reluctant potty kids I've known finally decided THEY wanted to be potty trained... I might have been play dates where they were the last - and - quite unkindly laughed at that finally motivated them... For one of my grandchildren, he FINALLY decided he'd like to play soccer - and M. (wisely) said, "NO WAY are you playing soccer in diapers!" Said grandchild opted to sit out one more season (in diapers, of course) before choosing to potty 'train'. I put train in quotes, because kids that wait till THEY are ready tend to be in diapers one day and full-fledged potty goers the next. (Thank heavens, no?)
SOMEBODY has a book about the last leaf on the tree that was afraid to fall (theory being some kids fear letting go of ANY bodily emissions)... It's a good book and could help... (wouldn't hurt, I suppose)... The librarian will know what I am talking about...
And - heck - make sure your sons knows it is HIS problem - and that his odors are offensive to you - and - change him at YOUR convenience (not his) and - well - be glad this too shall pass!
I predict before the year is out (2009, I mean) he wil finally be done with being a baby. Don't waste any motivating moments to point out that, as a diaper-wearing baby he is not quite old enough for that activity - but - DON'T bother punishing or shaming - because - strong-willed kids are too smart for that!
Happy parenting - and - as the happy mother of 14 and 6-yr olds, you are well aware - this too shall pass!