I would have mine wear a coat if it's cold. You will get an idea what his temperature threshold is when he starts to shiver or curl up. Little Zulee likes going outside without her sweater, but she has lots of hair right now.
I am not so sure it goes on coat length as on dog. Mandy girl gets chilly and I can see her uncomfortable so she wears a sweater. She often wears it for days on end.
Casey doesn't show it but I put one on him if its really cold.
Casey has to jackets and one full pc suit for sticky snow play.
Mandy has a number of sweaters and a full pc as well.
I almost bought another today for Mandy LOL but decided she had enough pink sweaters.
I guess it depends on how long you are outside for and what he is doing. If he is running around outside alot and doesnt seem bothered by the cold then i would let him be. If you are taking a walk then i personally would put a coat on my boy. Walks can be cold and they arnt working up a sweat like running.
Both mine wear coats in the winter - waistcoats as well when it is below freezing. We do agility in the open in winter and standing around waiting your turn soon gets them cold. They also wears a Ruffwear waterproof jacket when it's raining. They don't have undercoats and the rain goes right through to their skin. Just recently, I have sent for 2 Hurtta waterproof fleece overalls. These will keep the snow from balling on their legs and they can go out for longer without getting elephant legs.
My Ginger just got cut really short and we live in Ontario so it's cold out now. She's wearing a coat outside and isn't out for very long. My vet/fiend encourage this too. I think it is more about the change in coverage for her & she is feeling the cold. The groomer actually said that this was the best time of year in Ontario to clip her back because she isn't out for too long and the house is nice and warm. She still plays outside with Keelyn just not for as long then she wants back in. The first couple of days after the clip we actually threw a blanket over her at night when the heat went down, and she stayed under there all night. It was easy to tell she was cold, normally she likes to sleep on her back, belly up legs in the air and after the clip she's curled in a ball, nose and toes tucked right in.
I think it depends on the individual dog and what it is doing while outside. Our dogs are in continentals with bare butts and legs and it doesn't bother them at all. But, they are usually running around playing; which raises their body temperature. They will stay out for one to two hours like that in 20 degree weather without any trouble. They have been clipped like that since before winter set in so they had a chance to acclimate.
I wouldn't leave them out without covering if they were just hanging around though as I am sure they would eventually feel the cold.