I'm currently dog sitting my brother's boxer while my brother is in the hospital. The dog is 3 yrs old, and has always been very thin. He has recently lost a lot of hair and is emaciated. We thought it was a food allergy, so we switched his food. He was also flea-infested, so we took care of that. What's the best thing to feed him to get him to gain some weight? Like I said, he's always been very thin, and my brother has been working to try to put weight on him for a long time.