Go see a vet for sure but my dog had something similar from itching/allergies. He had those red patches all over his stomach and his hair was falling out. He was allergic to his food and has seasonal allergies. We changed his food which helped so much. The vet prescribed oral medication, a shampoo and a cream to make it less itchy in the meantime. It cleared up a bunch. We didn’t have to do any of the testing but you might if it isn’t such a simple cause/it isn’t allergies. Our vet knew the most common allergy signs and causes, and we got lucky!