First, check him for fleas. Even if he has never had them before, or it is too cold outside, they could be brought in from other animals into your yard. If there are no fleas, he could have developed an allergy to something outside, or even to something in his food. There are several good shampoos you can buy at any general dept store that help with the symptoms that have oatmeal in them that can help without drying out his skin. You can even buy a product to spray on the irritated areas to help soothe them and prohibit biting. If you cannot get him relief, please, take him to the Vet!
My 10 Yr Old Jack Russell Keeps Scratching And Biting Himself. I Try To Keep A Cone On Him But He Works His Way Around It. He Also Keeps Scratching His Sides When I Take Him Outside, Any Ideas?
I agree with the previous answer from Iam3002. My Chocolate Lab suffers from allergies and hotspots and her symptoms are very to your Jack Russells symptoms. A visit to a vet would be best. I hope he gets better.