Was he malnourished when you got him? Assuming you adopted him. A Vet check to make sure he doesn't have anything else may be in order. If not would just get him used to his new environment, get him working or give him a purpose. Will help him get adjusted and will give him an appetite.

Think along the lines of human fitness. You give him alot of food he will start to put on weight but that weight may not be muscle. He will develop layers of fat. You give him exercise he will develop the appetite and start rebuilding his muscle loss. These hounds are built with lots of energy so need to get him out and run him ragged.