M.S.
My son officially potty trained at 3yrs 4mos. he was showing signs of readiness for months but he just was not willing to use the potty either. He is very strong willed and generally does not want to do what I want him to do :) So I backed off for a while and then suddenly one day at daycare he told his teacher when she went to change him that he did not want to wear a diaper and that he wanted to wear big boy pants now. He refused to let them put on a diaper, so he went commando for the rest of the day. We never went back after that! Sure there were some accidents in the first few weeks but that is normal. What I am trying to say is that that day HE made the decision, it was HIS idea and so now he was ready. Before then, I could lead him to the potty but I could not make him actually pee in it! Then he was real excited about being "big" - we made a huge deal out of the fact that he was a big boy now using the potty and wearing big boy pants. So keep your head up - if your boy is strong willed like mine you might have to kind of wait for him to decide and then run with it.
As far as your struggles now with getting a diaper on, would it be easier to use a pull up? I could get my son to stand up and step into a pull up but could not get him to lay still for a diaper. And as far as him not letting you change him, that could work to your favor. Let him sit in the poop. Maybe he needs to feel uncomfortable for a while. If he gets a rash, explain to him that having poop in his diaper gives him a rash and thats why he needs to either be changed or poop in the potty. My 3 year old got to where he understood that... and once he did potty training happened shortly after.