My 8 Yr Old Chocolate Lab Has Lost Over 10

My 8 yr old chocolate lab has lost over 10 lbs in a few weeks time. we took him to our family Vet who ran tests. Couldn't find anything wrong but gave him special food and antibiotic "just in case". It's been 3 weeks now and he hasn't put on weight. He actually looks skinnier. He pants a lot, drinks a lot but doesn't eat much. He acts depressed. I'm worried about my fur baby.