Try sitting him down and telling him the truth. It doesn't need to be blunt. Let him know that a freshly washed penis is more of a turn on then one that is not. You also have a legitimate concern you can pass on regarding not only his health, (the build up will make him more apt to get a UTI), but yours as well. You don't need a UTI or yeast infection either. As long as you are tactful and the conversation is just between you and him, it will be less embarrassing. If he doesn't care about either his own or your health, then perhaps its time to move on.

Sam2