My son is 6 and was difficult to potty train. About 4-5 times since he's potty trained he has started pooping his pants again. When his little brother potty trained last year he started doing it again. We had tried negative consequences and positive consequnces. We let him earn a puzzle by not pooping his pants at the time. A couple weeks ago he pooped his pants again and has been doing it again. It's not everyday but a few times a week. When he started again we asked him why he poped he said he wanted a reward. My husband and I both told him there would be no rewards only negative consequences if he pooped. I try to always make him wash them out and then he gets jobs on top of it. He's still doing it. How do I get him to stop pooping his pants without rewarding him.
Most potty issues usually work themselves out when the home environment feels safe and the parents don't take accidents personally. Each person really does want to have control over their bodily functions.
Have you had your son tested for encopresis. It is a disorder which sounds a lot like what you describe. If affects children over age 3 and adults, and there are treatments and cleanses etc. This could be something you might want to look into.
About him wanting a reward. He may just be letting you know what motivates him to try harder. This could be manipulative, and it could be him trying to motivate himself. You know him better than I do on this.
Have you had a planning meeting with him on the issue? Ask him what he thinks would help him, or motivate him to put his stools in the toilet and then see how he does. If he still slips up, he may need medical help.
If he is just lazy, but can motivate himself, then I would go with it until it is a habit to have bowl movements the right way.
I could probably talk about this at length, but this is a rough over-view of options I think.
Be Calm! God Bless!