There may be many things that are going on however just from the short description of a older cat who has haircoat issues, losing weight and a better than average appetite, an overactive thyroid (hyperthyroidism) would certainly be on my list. If blood work has not been done within the last 6 months, then this might be a good idea. Having your veterinarian doing a 'once over' would likely be a good place to start. Is his water consumption increased? Good luck!