I've started giving him a treat every time he comes in after I close the door while he's on and off the leash… so hopefully that will help in showing him that he will get a good treat every time he comes in. Today is the first day since Monday he's been outside off leash during the day, which is when we have this problem, so hopefully there will be a little improvement. I've learned with him that nothing happens over night, so a little improvement is better than nothing.
Today is warmer, still cold, but a lot warmer than it was earlier this week. We have a saying here, "If you don't like the weather in North Carolina, wait 3 days, it will change." So I don't really have to worry about him freezing, and can focus more on a little training.
My roommate, who owns the house, wasn't really all that receptive to the idea of installing a dog door. It's understandable though; one of her dogs is a digger and likes to dig holes under the fence, so she has to be constantly watched while outside.
In the last house we lived, we were about to keep the door open most of the year, and he came and went as he pleased for the most part. Unfortunately, it's too cold to do that here. I'm going to try opening the door when he comes to it, then walking away for about a minute to see if he will come in. Usually, if I'm standing at the door after he's made it known that he wants it open, he'll just look at me for a second, and then run off. Maybe walking away will help.