Hi, M. --
I am in no way an expert on potty training, though, to my daughter's credit, she was potty trained by 2.5 yrs... at her own decision. I know this is something that is EXTREMELY difficult to control, since it's really something the little people have to decide they want to do.
Do you happen to have a good travel potty? We had one that collapsed into a briefcase, which used Ziplock gallon-sized bags as "containers," and it was pure magic. We used it everywhere -- in the back of our vehicle, on the go, etc.
My suggestion to you would be to get such a potty - something of a size your son can control, that he can experiment taking with him under the table, in the corner, anywhere HE wants and feels comfortable with, even if he'll just sit on it, to see that he's in charge of this potty training thing.
We used to keep bubbles in the side pockets of the travel potty and our daughter got to sit and blow bubbles while she went. We tried to make it as fun as possible, and much of the time, that worked.
I know that no pooping on the potty is a common issue with kids, so I wish you luck in helping your little man feel comfortable about it. Somehow, he has to be made to feel that this really IS his business and he's in charge of it. We hounded our daughter way too much, trying to control it, and it made her furious and rebellious. She had to just decide she wanted to make the change, and as soon as she did, she was trained. She would even get up in the middle of the night by herself to go potty... at not even 3 years old.
Take care, and good luck!
H.