I don't know whether this will help, but how about putting a mirror in front of him while he's doing it? (not permanently, just to show him what it looks like to others.) He may be unaware that he's doing it, or not realise that it looks kind of gross.
My ear's hurt just reading your title, I hope you get some help. When stressed I have a thing about eating noises, I cant deal with them (even me eating). Hopefully someone will come along soon and help you
