Advice for underweight cat?

This is a long story so please bear with me.....about 2 years ago I noticed one of my cats was slowly losing weight.  He was 12, had always been thin and had a sensitive stomach (vomiting several times a week).  He had no other symptoms. So I went to the vet.  Hundreds of dollars later - they can find nothing wrong with him!  Their best guess was that he might have cancer and I could see a specialist.  While I love my cat the idea of treating cancer seemed extreme, so we went home to wait.  At that time he was eating Science Diet KD as my oldest cat had kidney problems and the vet said all of the cats in the house could eat it.

Months went by with no change so I started researching online.  The only hint I could find was a comment that cats who didn't eat well could lose muscle if they were on a low protein diet.  So I started feeding him canned food (my older cat wouldn't touch it).  This helps with his stomach (he rarely throws up now), but he is still gradually losing weight.  I'm feeding him Purina ProPlan which the vet said is good quality. He gets all he can eat. I've tried both adult and kitten food and some other brands but it doesn't seem to matter. So he is now 14 and so thin I can feel all his bones when I pet him.  He still acts happy and his fur is healthy so I'm at a complete loss.  Any advice is appreciated!

Have you had blood work run again in the last 2 years to see if anything has changed, or gone to another vet to get a second opinion? I would go see the vet again if you have not already, a lot can change in 2 years.

If everything still comes back normal, how often are you feeding him. Maybe he needs to eat more frequent meals.

Have you tried feeding him salvaged meat leftover from your family's meals? Most Americans waste alot of food everyday, if you gather those scraps and he likes it, maybe he can start putting on weight. I know when my previous cat had cancer, it would aggressively take meat from the table. You can lock that cat in the bathroom away from the other cats so they cannot take the meat from him.