I have an almost 10 year old boxer mix. Lately he has lost some weight. His ribs are showing and his spine sticks out. He looks emaciated almost. I feed him a good quality dog food (the stuff is $30 for a 40lb bag) and feed him twice a dayper his weight. He sleeps most of the day and only goes out to do his business. He has always been that way. His appetite is good and he always finishes his meal. No changes in behavior and is in good health. He has always been a thinner dog but he just looks bad to me. He is due for shots soon but I thought I would ask for tips on getting some weight on him until I take him in.
As the other people have suggested, definitely have a blood panel done on him. You'd be amazed at how much can show up on one - and it will help narrow down the possibilities. Another thing would be to bring a stool sample in to rule out intestinal parasites. Good luck!