Definitely make sure he isn't having a digestive system issue that may be making it hard for him to keep clean or clean up well. Wet wipes might be very helpful. Get him the wet wipes intended for adults (he may feel ashamed using baby wipes with would be counterproductive), and explain that they are for big kids and adults. I am an adult with medical digestive issues and it can be hard even for me to get myself fully clean using just toilet paper. If you have never used them, wipes are much more effective at getting everything and really cut back on the amount of toilet paper used. I do use TP after the wipes to dry and make sure I am clean. If you can emphasize it as an I'm growing up rite of passage thing so that it feels special, maybe he will have more interest in cleaning up better. However, if it continues to be a problem there might be something else going on. With the ADHD symptoms in the family, the others symptoms/behaviors you mentioned, and a lack of interest or possibility understanding about the importance of personal cleanliness, it sounds like he might fit some of the diagnostic criteria for Autism Spectrum Disorder (more specifically the set of criteria that used to be called Asperger's). It may be worth looking into with a professional specializing in Autism Spectrum Disorders if the issue does not improve and if personal cleanliness becomes a bigger issue as he enters puberty. In the meantime, pick up some adult diapers for men that look like normal underwear (or some fun pull ups style diapers for big kids - medical supply stores might have these if not the grocery store). Many adult men of all ages use them, just like the wipes. There is no shame or embarrassment in this, same with the ones for big kids. ASD and many chronic digestive illnesses are disabilities, so it is important that using these products is treated as a shame-free, normal thing. Not everyone buys the same toilet paper or fabric underwear, these products are no different from that.
Okay, thinking like a kid . . . have you had a discussion of proper wiping or tried the flushable wet wipes? I mean, many a grown man will have brown streaks. Wiping can be an art. Look into encopresis. but also, discuss exactly how to do it and see if this helps. good luck
My 12 year old son has the same problem , constantly finding his underpants stained and now he's hiding them , I found two pairs in his wardrobe. I get so annoyed with him , I spoke to him about self pride and that people can smell it ,it's like he doesn't care . I even took him to doctors and she said he's lazy that's all. So what do I now .i then got his older brother to talk to him and he asked him to go to the local supermarket to buy cleaning items for himself and encouraged him to buy wipes deodorant etc. His brother constantly asking him have u been to the toilet since that I haven't found many dirty underwear and he is taking notice of how he looks . Although he still very messy
Also, if he has constant stains on his underwear, you might be seeing the results of encopresis (though pinworms sounds pretty correct). Look up encopresis and see if it sounds familiar. If that is what is happening, you might get some fiber gummy chews and give him one every day. It makes the stool an easier texture to pass and might prevent him having the blockage-and-leakage problems associated with encopresis.
Also, try the "moist wipes". They are more comfortable to use and do a better job.
Oh, and if you have any AD/HD questions feel free to ask me over here -
http://www.medhelp.org/forums/ADD---ADHD/show/175
Have you treated him for pinworms? That causes this digging behavior - it may be unrelated to wiping.
You can get pinworm treatment over the counter at the drug store. It's in the lice section of the store. There are ways you can test for pinworms, including checking the child for worm eggs at night but that sometimes doesn't catch anything, and there are no warnings on the pinworm package that treating is harmful.