What can I do to help my sick dog?

He is a 11 year old chihuahua and is usually very healthy. He hasn't thrown up or anything, but he keeps climbing on the T.V. stand and crawling under the beds and is just acting sick. He won't eat either. We changed his food, but that was was a couple of weeks ago and we changed him to a better quality of food(we went from kibble and bits during the day and Cesar's at night to Blue Buffalo) and it hasn't affected him until now. He sometimes gets into bird droppings and once got a virus(or bacteria, I don't really know which) that nearly killed him and I'm concerned it might have happened again. But he also gets sick when he gets table scraps, but not this bad .Should we take him to a vet or see if he gets better? Is there anything we could do to make him feel better with out taking him to the vet?