Perhaps there was something that scared him during that last good morning walk you had with him. (The one before he locked up.) A car back-firing, or some other scary thing. The reason I mention this is that one of my dogs protested going out for the nighttime walk. He was fine during the morning and afternoon walks, just not the nighttime walk. The culprit? Fireworks a few blocks away. Just once. But the phobia developed. His resistance went on for months after that one night. I would coax and plead with him, and eventually, I got him to go, but he was nervous and wanted to get the duty done and get back to the house. He was better when DH would go with us. As time went on, he got better. Now he is fine, but i worry about fireworks (the cussed things!) in the future.
I can't imagine how cold Michigan weather can get, so I can't comment on that. Another thought... Check his paw pads. Do they look dry, cracked, irritated? Perhaps he needs some protection on his paws.
Whether it is physical or emotional for him, hang in there. He needs to relieve himself before you leave in the morning, and he needs for you to help him work through this.
Good luck.