If you live in a climate that is not his normal habitat (check your breed's background - jack russels can take some cold to go out and do their business, but not to live outside, they were bred to go to earth and dig, and that activity keeps them warm and no one digs in frozen ground, by then they are sitting or lying warm by the fire) (poodles also are not for the frozen areas) you'll need to help him stay comfortable. Most dogs love being warm when it's cold. So it goes to how you introduce the sweater. My first response was going to be it will keep him warmer and he might be annoyed. You need to get a sweater that is pliable and not scratchy and fits comfortably but is not too baggy nor too tight. Try just putting some tshirts on him at home to get him used to the feeling of clothing going over his head and onto his front legs. Take them off after a few seconds and keep extending the time while you hold the dog and pet him and even give a treat for letting it stay on him longer. Then when you get to the sweater it will be a tailored fit that will really work for him. Mostly he shouldn't mind with some help from you. It's not 'unnatural' except that they don't get to pick them out. We've been helping man's best friend face the elements comfortably for many centuries. By the way, if the dog is shivering when you take him out, he's cold. If it's a snowy climate, keep checking his pads and pull out snow clots before they become ice clots between the padding on his paws. Please go for comfort for your dog, not just what 'looks' good to you. You want to keep his chest warm and dry as well as his back, so a doggy turtleneck can work for him. Enjoy!