It sounds like Nico has something going on that needs to be investigated. Weight loss with a good appetite plus small, hard stools usually means hyperthyroidism. I would also worry about kidney disease or failure, inflammatory bowel disease, a condition called megacolon, diabetes mellitus ("sugar diabetes"), and cancer. I can't recommend a diet without know what's causing his problems, because there isn't any one food that will help with all of the potential diseases. If you ask your vet to do bloodwork (an organ panel to assess Nico's liver and kidney values and look for diabetes; a complete blood count to look for signs of anemia, infection, or cancer; and a thyroid hormone test) and the vet won't do it, then you need to find a new vet.