If you have an area where he can get warm if he needs to and he doesn't want to go in, then he is warm enough.
I keep two dogs in the yard when I'm at work. I'm in South Texas, so it doesn't get that cold, but it's all relative. My dogs aren't used to really cold weather. I have three igloo houses that I put hay in when it gets cold (they'll take blankets out and run around the yard with them). Rarely do they ever go in. The igloos are on my covered porch, so it keeps them out of the wind and rain.
However, if we're expecting bad storms and it's really cold - I'll kennel them in the house. More for my benefit then theirs.
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