Have you tried the wet wipes idea - I know you get ones for kids.
Please don't ridicule him for it. He may not seem to mind but maybe that is because he doesn't know how else to deal with it. He may deep down be embarrased and annoyed at himself. Believe me, you don't want him to stop going to stop the problem!
Try the wipes, they may work and if they don't, stop making an issue of it - it will sort its self out when he gets a girlfriend!
Thanks Debbie for agreeing - its really great to know that you are doing something right yourself once in a while - no matter how much we feel its the right thing to do - it is reassuring when someone alse says so.