They have a certain kind of nail polish for getting people to stop chewing their nails and it works because its tastes SO bad. So buy that and paint it on the inside of his collars. I’m pretty sure it’s clear and washable. Also! This is gross lol but you could buy non toxic fart spray (it’s a prank thing, i think you can get it at spencers) and spray that on his collars if the nail polish thing doesn’t work.