Hi K., you should be teaching him to wipe himself now when he is at home. It might make it easier for him if you get him some "Kando" wipes and put them in his backpack, and talk to his teachers about his anxiety about the issue and tell them you are sending Kando wipes in his bag so that he can wipe himself. It might help him to gain the independence he needs.
On the other hand, have him start wiping himself at home with you there. If he doesn't want to, then ask him why. Is he afraid that he won't do a good enough job. Does his but hurt if he doesn't do good enough? Where is the anxiety coming from? Unless he is having irritation from not wiping good enough, why is he having someone else do it for him (I know that it isn't sanitary, but he will get better, just tell him to wipe until the tp is clean). Let him know that it is okay if he has racing stripes (I wouldn't call them that, he might think that it's cool) and you know that he will get better at doing him himself, and tell him how proud you are that he did it all by himself. If he doesn't do it good enough at home while you are with him, don't clean up after him. Don't be tempted to say "good job buddy, now let mommy make sure you got it all". Just let it go, and buy some bleach and more underpants. If he says something about his stripes, so something like "oh, well did you remember to wipe yourself until the tp was clean?" '
Good luck. I still think it needs to start with asking him why is doesn't want to wipe himself. or at least by saying "hey lets learn to wipe your own butt todya".