There are several things you can do...just depends on how resilient you are.
First let him sit in it for a while when it become seriously uncomfortable...he'll stop.
Second...put him back in diapers. This (if he has friends) will humiliate him out of poopin' his pants.
Third...how about lettin' someone else try to potty train him??? Sometimes a different influence works better than our own.
Fourth...have you tried the potty training books??? Those are great and funny! You'll be entertained while he is learning. These books are great kept @ the potty chair only. Give him some readin' material...he's like ANY male...(BIG smile) he might develope the habit of readin' in the potty...but who cares??? He's gonna accomplish two things...Goin' where he should...AND become a good reader. Both are goals you want him to succeed in.
I realize that the first two suggestions seem a wee bit harsh...but they are proven to work. My own son was a piece of cake to potty train. However I have friends who struggled and were so weary by the time it worked...they were at a point of they didn't care any longer. When their boys DID get it...they threw a party!!! LMAO
Those first two also were suggested by Dr.'s. You just watch and make sure he doesn't get a "diaper" rash and he'll soon be uncomfortable enough that he'll stop goin' in his pants and do the correct way.
I had to come back and add this...(after readin' other responses).
I too used the Hot Wheels/MatchBox cars as incentive.
What we did was when we went shopping (they sell those EVERYWHERE)if he told me when he had to go...and REALLY had to go...he got a car. There are SO many cars out there that you can go for ages without duplicating them. I really have NO qualms about spendin' $1 a store if it means no soiled clothes. You just limit the number of stores you go to.
I believe we STILL have all those cars (he's now 16 soon to be 17) and I won't give them away or sell them. His kid too will need to be potty trained some day.
Good luck and keep your chin up.