Here I am with Mrs.Zardoz in my early days of kilt wearing at Sunspot in Winter Park CO. I don't recall the altitude there but it is the highest point in Winter Park, over 10,000 feet. The temp was about 26- 27 degrees, sunny with a light breeze. In the picture I'm wearing a "old model" acrylic Stillwater standard, good wool hose, and a chamois shirt with a thermal undershirt, and a wool felt hat. Shelley as you can see, is wearing everything she owns!
Since I am from the tropics, (Houston, where it last snowed in 1972) and was unsure how warm the kilt would really be in this situation, I started off the day with a big coat and some thigh length thermal undershorts. As soon as we walked from the chairlift to the lodge I stowed the coat and the shorts in a locker because I was too hot. Looking back, I think that acrylic kilt was warmer than my 13 oz wool kilts are. In any event, I think if the fellow in question is going to be wearing a heavy mil-spec kilt as was mentioned, and good kacket and hat, I don't think any extra undergarments would be called for. Just be sure and do a good sweep before sitting on any outdoor benches!