A severe case of Lupus would mean he'd need to cover up in the sunlight. This condition lead to the Vampire & Werewolf myths (see derivation of lupus).
On the otherhand, as with the scantily clad "ladies" in the depths of winter, he could merely be insensitive to climactic conditions. My neighbour wears the same black jumper & black coat all year round. He's either got a wardrobe full of the same clothes, or he never changes. I try not to stand close enough to find out...
When I was in my teens I never seemed to feel the changes in temperature. I would cycle in shorts and a short sleeved top from March to November and then just use a lightweight jacket and leg warmers for the coldest days of the winter. I wouldn't, even couldn't, dream of doing that now. Shorts may be the order of the day at present, with the heatwave, but I soon wrap up when the weather changes.
Have you ever thought of re-positioning your PC? No? Join the club!! Scholgirls have been doing this for generations - it's known as pushing the 'fashion' boundaries to see how far the school rules will bend.