At least you know he will use it for shelter, that's got to make you feel better. My feral, that over the past few years has become quite tame, she will not touch dry food. She will not touch good canned cat food either. She will only eat fancy feast, and only certain flavors and varieties. She will eat people food though, that she loves, probably because she ate most of her meals from a trash can in my neighborhood for years.
I can't imagine those kind of temperatures. I'm in Florida and we are having cold temps, for us, in the 30's at night, and people and pets here are not used to those temperatures. I've been lucky enough to get Arwen, my feral, to sleep in my garage in the evenings. As soon as the sun comes up, she is out the door and when the sun really starts to shine, she is curled up right in the best spot she can find, funny how they do know where those places are.
Cats are such resilient little creatures. If only they knew how much we worried about them, especially when it's cold. Good luck with your ferals, I'm betting they will use the places you have for them.