A.C.
I wonder if you could put something yucky tasting on an old shirt collar and let him wear it? I would use and old shirt so that you don't get stains on good clothes. Maybe try lemon juice or vinegar. I have never heard of anybody doing this on clothing but it is recommended for thumb sucking. Just a thought.....
One other thing I thought of is sewing the coarse side of velcro to the collars of old shirts. I don't think he would like the feel if chewing that. Maybe it would break the habit. Never heard of doing it but it's an idea!
Also remember to use positive re-enforcement. Maybe praise him or give him a small treat if the shirt is dry and not chewed!