Sounds like my son. We found out a few years later he has ADHD, anxiety issues and Asperger's. It also turns out he has sensory issues (common with ADHD) and honestly couldn't feel when he had to go, or the poop on his butt after having an accident. And he simply didn't care whatsoever about the whole thing. He ended up with encopresis (uncontrollable soiling due to chronic constipation - lots of soft poop oozed out around hard lump so it didn't look like constipation... he was 'going' almost everyday!).
Anyways... please take him to a doc to rule out encopresis (usually an abdominal x-ray to see if he's impacted). Then, get him on a schedule. Forget pooping for right now, just get him to sit on toilet a few times a day - 20 minutes after a meal (when the body's involuntary muscles release to let poop out) and at bedtime. After a few days, make him do clean-up. No, he's not too young. Help him a few times and then just standby and let him do it. Have him rinse out the underwear and clean up the mess. It won't be long before he figures out that it's a much less hassle to stop playing for a few minutes to poop then go through the process of cleaning up, putting clothes in washer, starting washer, getting clean clothes and putting them on, etc.
As for motivation, it took a long time for us. We ended up using poker chips - 1 for pee, 2 for poop, 2 taken away for accidents. Everyday he got 5 chips, he got to play on the computer or Playstation2 for 30 minutes (he was thoroughly addicted to Lego Star Wars at the time). After a week or so of consistently getting it, we upped it to 10 poker chips. None of the normal stuff worked. This worked because he loved it and it was the only way he could play those games.
Every kid has a 'currency' - you just have to look. Make him earn every minute of TV time - that works for most kids. The problem for most families is that the parents aren't willing to turn the TV off to enforce it. It won't work if it's something he has access to at other times. Take all the toys out his room or toyroom and make him earn 1 back everyday when he gets to a certain of chips. Take away a favorite sweatshirt or stuffed animal so he can earn it for bedtime.
Do you have a potty ring for the big toilet and stepstool? It's pretty darn hard to poop when you are balancing and concentrating on not falling in. Also hard to push if your feet are dangling.