A couple questions come to mind; what food are you feeding now? How much are you feeding now? Is your dog extremely active, moderately active or not very active? Has he been neutered? When was the last vet exam? Fecal exam? Is your dog on monthly heartworm prevention?
All of these are factors in determining a proper diet for any dog. I recommend feeding a name brand dog food not a store brand. Feed measured amounts of food twice a day. If you don't know how much you are feeding now, use a measuring cup to determine the amount. You may just need to increase the amount offered for daily rations. If your dog is very active he needs more food than a dog that is not as active. He may be burning all of his calorie intake with his activity. If he is not neutered, his weight may fluctuate due to hormones, especially if a female is "in heat" within a 5 mile radius of your home. Has he had a vet exam within the 6 months to a year, is it possible his weight needs to be addressed by his vet? Has he had a recent fecal/stool exam to rule out intestinal parasites? Is he on a monthly heartworms prevention that also deworms for intestinal parasites? My dog has to be on a heartworm prevention that covers whipworms, she almost died from them last fall. They caused a lot of weight loss in a short amount of time.
If all of these bases are covered and increasing food does not help, I recommend rechecking with your vet.
Good luck and best wishes for success with your pup!