If he's hacking and gagging a lot, I would not use a collar period. The only time I use a leash/collar with Jackson (who is almost 13lbs) is when we're out back on his long retractable leash where he doesn't pull. On walks - he's wearing a harness. He doesn't pull on walks, either but I do find him easier to control with a harness rather than a collar.
If you're worried about it matting/damaging his hair, a member here on YT makes SILK step-in harnesses that I've heard nothing but GREAT things about. Because they're silk, they don't matt up the air.
http://www.yorkietalk.com/forums/yor...elections.html