Cat is losing weight although he eats well?

My 11YO male cat has been losing weight since January. Blood work done reveals a very healthy animal! He has also been licking fur off and is bald on various parts of his body. He's active, alert, rambunctious...normal...but looks so skinny. Vet has done check up and everything seems normal. He may have a food allergy; vet says if he's not bleeding from the licking of fur we can leave him be...he does not recommend cortisone to relieve. He also urinates occasionally over the side of litter box. Kidneys are good. We have another cat, male. They've been friends for seven years.