I used M&M's with my daughter which worked well. She literally had 2 accidents after starting. She was nearly 3. My son has allergy issues so that didn't work for him, but he was so facinated with wiping, flushing and washing his hands at about 21.5 months but we wouldn't let him unless he went potty so he started going. He will stay dry many days if someone mentions the potty frequently enough. He can get caught up in playing or just forgets and will have accidents, but I attribute that to his age. He will not poop in the potty though. Again, He's young so we remind him to tell us when he does in his diaper, but can't really get too upset.
One thought - I know many people have strong feelings about this - we basically went from diapers to underwear and skipped pull-ups. I'm not super strong on the subject, but I do think you should decide what you can live with and stay with it. If the washer full of soiled underwear leads you back to pull-ups, then perhaps you need to stick with pull-ups longer to go to diapers until your son can establish some consistency with staying dry. Try making a chart or marking a sticker on the calendar for each dry day. Tell him that after a week of staying dry, he can try underpants again. The only times I resorted to pull-ups with my daughter were naps/bedtime - it simply takes a little longer and I am pregnant and not interested in changing a full size sleigh bed that is up again a wall on a regular basis - call me lazy, and time we'd be away from a potty and getting there within a reasonable time would not be possible -in the car for a long trip, airplane, etc. I know some think it's cheeting, but I'd always explain to my daughter why we were using a pull-up and she did well with it. She'd ask for a potty first and was reluctant to go in the pull-up, but I would tell her when she asked for the potty that it was ok to go in the pull-up because there were not any potties around. I didn't see value in her ending up having an accident.