• Military weight kilt - What Price Glory, surplus, eBay, that retailer Rigged posted the other day ...
    Heavy weight wool hose, unrolled up over the knees as necessary
    Loose underwear, that will cover but allow ventilation, if he decides to go that route.

I've spent the last few winters out and about in kilts, sometimes a couple of hours at a time, and the only times going non-regimental would have been more comfortable were when I was wearing a non-traditional kilt (UK or 5-yd PV).
NOTE: I've been comfortable in a 5-yd, 13 oz kilt in 20°F/-7°C weather.