If he has stopped chewing (though like Tunes said, once they start, they don't usually stop) the fur will grow back on its own. It will take longer to grow back than say if he had slipped fur because it has to shed out naturally which takes some time. (Slipped fur grows back pretty quickly.) To speed things up you could theoretically force him to slip the fur by grabbing a small bit of it and just holding on, though I would not recommend it for a large area. And more than likely he is still chewing so he'd just give himself another bad haircut as it grows back.
Chins that chew tend to be more easily stressed animals, but other than that, like Tunes said, its just cosmetic and doesn't make him any less loveable. Just enjoy him for the unique critter that he is and tell your friends he does his own haircut.