I need help with my old dog :(?

My dog is 8 years old. & he's getting old I think. Is he sick? He barely eats, he's always shivering. Like always. There's not one minute he's not shivering. But he's a happy dog. He just has those symptoms. But what's wrong with him? We're getting worried. We try warming him up but he never stops shivering. & we gave him food he loves & he won't eat it. Help I want my dog back