Sorry, I'm not answering your question, but truly - I don't think you have to worry about it, unless he's poorly or old. Our shorthaired cat has her own cat flap and goes in and out at will, and willingly goes out, picking her way through snow up to her shoulder, when it's -20 C (around 0 F?) out. She just comes in a bit sooner, but still, she's out there for maybe half an hour, 45 minutes with no ill effects.
Quite funny, she comes in with all her fur and her tail poofed up, generating her own "down parka" so to speak
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett