We bribe with tickets for Wii time (my son's current passion). Each ticket is worth 2 minutes. He gets them for brushing teeth, combing hair, washing hands, etc. We're supposed to reward immediately for the best effect, which is easy because he reminds us. :)

You may want to check with an occupational therapist for sensory therapy for vestibular (sense of position in space) issues. Balancing over an open space over cold water may be a challenge. Tactile issues may also be there... he may not truly feel the mess on his bottom afterward, or he may be repulsed by the feel of wiping or the idea of having his hand so close to the mess.

Or, he may indeed be distracted and this just not be such a big issue to him. In that case, as EthanMom has suggested, a visual checklist, guided practice through the whole process for a bit longer, etc. -- anything that will help increase the value for him of a clean bottom -- is something you might want to try.

Video's helped us...my son loves to watch them and do what they tell him to do..like brush his hair and teeth...elmo I think had a potty video that worked great

I think a visual schedule too..break it down in steps. you can even laminate the page with a box beside each picture and have him check off each step with a dry erase marker...and give him a treat for completing the task completely by checking all boxes. I am still learning myself and my son is 8...as well..and still working on toliet training.

I'm not sure if this will work with an 8yr old the same way it does a 4yr old:

1. Make up a silly song about wiping. We have a silly song for tooth brushing and that seems to do the trick.

2. Have a 'task chart' (easily made with velcro and cut out shapes of toilet paper rolls or something fun looking) that will serve as a visual reminder. Maybe he can earn a small prize/priviledge for filling the chart.

3. If the no wiping leads to icky underwear, have him clean up the mess.

...good luck :)