Fall is upon me and I'm trying to figure out when my dog is going to be cold- his ears get cold inside when he is cold I think- so that's a good predictor.

He does not seem to have any type of undercoat- he is some sort of hound mix- is that possible or am I just not seeing/feeling his undercoat?

It is definitely short and straight. He likes to be outside walking, running alot- should I be worried about him being cold? When it gets to be like 20-40degreesF then should I worry?

Someone at the dog park gave me a fallish-type coat that a friend had given them that was too big for their dog- it's not too Paris-Hiltony of me to make him wear it? I think I may get him a warmer one for the winter.
I don't trim it- it is about 3/4 an inch long and he sheds alot...