I want my older cat to gain weight?

he's become so thin in the last year. he's always been very fluffy and fat- but his health seems fine, i'm just uming his weight loss is due to the natural aging process. my question is whether or not there is a cat food i can start giving him to plump him up. i've tried wet food and treats but it just seems like nothing is doing the trick. i have dry food out for him to munch on when he feels like, and he's definately still eating. anything i can do?