One of the rules of outdoors is this: "If your feet are cold cover your head."
The body sends warmth to the most important parts first, namely your brain. If you are not warm and insulated enough your feet and hands will get cold first. Also, since a lot of the warm blood is going to your brain, if you don't cover your head and neck well then that is a really easy way to get chilled. I would say make sure his head and neck are covered, as well as his midsection. See if that makes a difference as well as the wool socks.