Feeding to gain weight

I have a full bread Maincoon he is 13 years old he has a hyper thyroid and i give him 5mg of tapizol daily. Imin need to find a type or kind of foods that i could get him to gain weight because he eats good but doesnt gain any weight if anything he has slowly been losing weight for the last year to year and a half to the point of getting very boney and skinny. Any ideas to help gain weight!

Have you tried good quality kitten food? Ask your vet is that's okay, it's higher on calories but also on protein. If there are no other restrictions, I would just spoil him with healthy treats and gourmet cat foods.

You can also give them some Nutrical. It has all of the essential vitamins and proteins. If he is not on a special diet, you can also feed them kitten food to put weight on.