Sure got us stumped Hang in there, someone may have the answer yet. Hope he gets better soon, you're doing all you can and that's all you can do. Keep us posted as to his condition...
Thought frostbite too until I read through the thread...
It's been asked, why was he originally brought into the house, his foot?
Does it get "chilly" in the area he is being kept?
When & how long has this condition been happening?
How is he doing; eating, drinking, pooping, activity level?