Extra Layers for Lassie

Do dogs just naturally tend to stay outside longer when the days get shorter? My dog Rocco refuses to come inside and makes himself cozy and comfortable sleeping on a weathered couch every night. His routine of dozing by my bedside is no more but instead growls at me if I budge him from his new favorite dog house - the garage. It's nice to see that he's getting more use out of a crummy couch destined for the landfill, however, I miss him nosing me at 6am and whimpering me awake during his occassional nightmares. Worse yet, how safe is it to leave a dog sleeping in 20 degree temps? I presume dogs suffer frostbite but how cold does it need to get before that happens? I could entice him back inside with a bone or biscuit but minutes later he's always whining for the door. Might my aging and gimpy Golden want privacy because he knows something that I don't? I just got him, surely, he has another 3 or 4 more years of walks, talks and barks in him. Then again, I admit, I've not been good at keeping anything around, especially the opposite sex, for more than 6 months - given the rate of speed that I live that should be of no surprise. Perhaps a portable heater will help ease my worries when it falls below zero but until then I'll just keep covering him up with a sleeping bag.