If he consistently avoids dressing warm the human body is pretty adaptable to cold conditions so I wouldn't look to deep at that. Both a persons circulatory pattern and metabolism change when exposed to the cold, and people who expose themselves to cold consistently enough respond in far better ways. Their metabolism will shoot up near immediately when someone not adapted will only gain that after they are already cold and shivering. And blood flow is maintained to the extremities but just avoiding more of the skin's surface, where as the unadapted will have just a general decrease in bloodflow to that entire extremity.

If you go extreme enough humans can even walk barefoot through the snow without a problem all day without a real problem, where as someone who wears socks and shoes when it is freezing cold will get serious frostbite on their feet in like 30 minutes or less if they tried it without adapting themselves over time.

For a direct application of this, ice climbers will soak their hands in ice water for 45 minutes every day in the weeks leading up to a climb so that their hands don't freeze and maintain blood flow when on an ice climb, because obviously you can't just stop and warm up your hands by a fire when you are halfway up a frozen waterfall and having stiff or frostbitten fingers makes climbing more difficult/dangerous.