yeah, the wet wipes is a good idea.

We had the same problem with SS when he was that age (he's now 14). Here's what we did:

1. Made SS scrub out his skid-marked underwear. If I found it after he went back to BM's house, DH had me set it aside and SS would scrub it first thing when he came to our house for visitation.
2. If I found it while SS was at our house DH made SS shower and then SS had to scrub the marks out.
3. Like LMKAH_HMBF suggested, we have baby wipes right next to the toilet paper. In fact DH asked me to keep buying them because he uses the wipes too (LOL). I think we all do now - IMHO - you just feel cleaner when you use the wipes.

It took awhile but when SS realized he was responsible for cleaning his skid-marked underwear he changed his ways.

Hope this helps.

~BettyRay

My friend's son had that problem. What they did was made the boy open his piggy bank and buy new underwear whenever it happened. I honestly didn't think that was the way to go; I would think making them clean it themselves would be better, but this is what they did!

I say wet wipes... kids are gross, aren't they? I had this issue with my BD. They are in such a hurry to get off the pot, they cut corners. Nasty a$$es. (literally)

Uh, yea. I have a skid who is 9 and still has the skid marks. In fact, one day there were two brown spots on the carpet in the bedroom skid sleeps in-guess what the spots were?!! So nasty. The kid could have at least mentioned they were there, intsead of possibly staining BRAND NEW CARPET. I was the lucky one who found the skids in the underwear later when doing laundry. disgusting

Good luck! I started making him clean them by hand because IM FED UP! Another checkmark for him to add to the reasons why he hates me....OH WELL! 

My SS10 did this when I moved in. He was 6 at the time and would shit in the downstairs bathroom and not flush and there was never any toilet paper in there. So I had to show him how to wipe his ass and flush like a human. I always judge my DH and BM for that one. Her more so because she was supposed to have been raising them while they were young because she didnt work. I like throwing that one around now when I get pissed, 'I am someone in your childrens' lives! I taught SS10 how to wipe his ass!' As for the wipes, I agree. 

I taught my kids to just wipe until nothing is left on the TP. This is gross but I used to think all men for some reason pooped their undies because I knew my father did, my grandfather did(I did laundry growing up), my ex did, and I've heard many women complain about their husbands doing that. Then I got remarried and my husband's undies never have skidmarks and when the subject came up one day he thought it was disgusting so many men don't wipe themselves. That's how I found out it's not a normal thing, it's just gross and lazy and not all men do that.

Out him to his peers in a diaper until he figures it out.  It will take exactly one time having to wear a diaper to school and be ridiculed and humiliated by his peers for this to cease being a problem.