How do you know? if his underwear is marked you can explain it is hard to wash them,if it only a bit let it go,just give him fresh underwear each day. Check he has paper to use at school and washing hand facilitys.Dont admonish ask him to be more careful in cleaning himself.
any other problems with disobedience? I guess he doesn't want to get his hand dirty- is it at school time? Maybe they don't have soap or paper towels or give them enough time for handwashing? just brainstorming along with you