If you can do it in a matter-of-fact way, start having him help you with the clean-up and the laundry issues involved. "This is what we do with poopy underwear." Then take him through the stages of disposing of it, cleaning himself and washing his hands well, rinsing the underwear, washing it (with everything else that has to go in the washer so as to not waste energy), drying and folding and putting in the dresser. The trick is to do it without being punitive. Make it the same as "This is how we clean up when we spill juice" or "This is how we sweep the floor when we drop or break something." When his sees how long it takes and how many fun things there aren't time for, he may take it as a lesson in being more careful. Just as spilling the milk is an accident, we can still take steps to prevent it - not pouring too much in the glass, using 2 hands, etc. Same with going poop - we can make an effort to use the potty, or we can clean up when there's an accident, etc. Another thing you can try is books - not just the ones on potty training, but something kid-appropriate on how toilets work, where the waste goes, etc. Some kids find that fascinating, and enjoy using a "system" they understand. Good luck!
