M.P.
My 3 1/2 year old had a hard time going poop in the potty and woke up one day and told me he had to go potty. So we went in the bathroom and he pooped in the potty like a big boy. His father and I are seperated so we made the point of calling his father while at work to tell him and his grandmother and even his aunt who took him out and bought him a special treat for being a big boy.
You don't want to force him because it may revert him from using it at all. I know it is frustrating and hard to deal with but in time he will do it on his own. It took me just setting my son on the toilet from time to time. Even at his age now he will request I go in the bathroom with him from time to time when he has to go. It might be the one on one time of you being in there.
I was preganant with my youngest while my son was potty training so I told him that I needed him to be a big boy for Mommy so when his brother came then he could help me. It seemed to work very well. He didn't like the idea of being a "baby" anymore.