How old is Joel? I don't think it's been mentioned. If he's very young and small, he could be going a bit hypoglycemic, it's really common in tiny young Yorkies. Keep some Nutrical on hand. Also, I would get him a coat for sure, to keep him warm.
Check out AlaskaYorkie (YT member) and his dogs - they live in Alaska too and have, like, the greatest life EVER.
I am sure Mike would help you with raising Yorkies in AK.