How can i fatten up a 13and1/2 year old Tabby?

at this age i would suggest slowly switching him to a good food.

No. Iams is not a good food.

Go for something with less corn and less filler in it.
Try something like Dr Weil or if you have the means, use google and learn how to feed a raw diet.

At that age the digestive system might be beginning to give problems so the easier the food is to digest the better it might be.

Of course I am no vet. I am only speaking from my own experience with my 10 year old cat. She was fed purina foods her whole life. I googled the ingredients and changed her foods right away. I truly do have a whole new kitty .

good luck