Not sure if this is even close to the problem but are you sure its the cold thats making him shiver? I have border collies for working dogs and I have two that will work in any weather possible, no problem. When its time to work certain sites though and they get super focused, they'll just sit there, focus and shake. It looks like they're shivering but they are getting so amped and ready for their command to go to work, they shake. Just a thought, good luck