I've read, especially for cats in harness training, if they lay down it's fine. Take it off for a while. When they become accustomed to it being on, take it off for a while so they're not consistently bothered by it. Also, you can put it on, and feed him treats IMMEDIATELY. I dunno if that's what you're doing. Distract him as much as possible. You might also want to consider a harness, instead of a collar. Collars pull at the neck. It's possible the leash is heavy and he can feel it tugging at the collar and the collar tugging at his neck. At this point you should probably get a lighter leash, or invest in a harness so he doesn't potentially feel like he's being choked.