Rub fish oil on his paw pads every couple of days? Doubt there will be any left-over for him to spread around the house. Then do it and try to clip one or two nail. Gradually work up.

I think cats are supposed to dislike moth-balls. You could set bowls of them around the couch. And there is this thing called a 'tattle tale' that you can put on your furniture. It's motion-sensitive, and emits a loud shriek when disturbed. Maybe get two or three.