You'll be fine. Talked with retic shippers about this not too long ago when my shipment came in freezing and the snake had a registered body temperature of 68 which felt freezing. A retic can sustain a body temperature quite low before anything bad happens. That sad, as long as you're warming him up slowly you'll be fine. DO NOT soak him to try and warm him up. Put in room temperature water and not much of it for him to access at his leisure. Most retics will feed the day of arrival, but give him time to gradually warm his body temperature up, then offer food. Cheers on the new kiddo arriving safe.