kitten food for an adult cat

my cat is coming up on 2 years old and is still eating kitten food because it’s the only thing that keeps weight on him. he’s a totally healthy cat, with good teeth and up to date on all his shots. he’s just a grazer. i had him on adult food for a few months but he was losing weight and when i switched him back to kitten food he got back to his normal weight of about 12 pounds (he’s not fat he’s just a long boy). is there anything wrong with keeping him on this food? the earliest vet appointment i could get for him is three months from now, so i was just looking for some insight! i can describe his current condition if need be!