He has to go on a diet.
Trying to explain the to him will go something like this:
Me: Hey bud, I have some bad news.
Winston: Oh god what happened to my pillow!
Me: Nothing, your pillow is fine.
Winston: WHY WOULD YOU SCARE ME LIKE THAT?!
Me: There is something I need to tell you and you are not going to like it.
Winston: Well I mean, if my pillow is fine I don't see what could possibly be-
Me: You have to go on a diet.
Me: I know, it sucks but the vet says you have to lose weight or else it will hurt your tiny tiny bones.
Winston: I see how it is, I get a little bit of holiday weight and you don't think I am handsome anymore.
Me: Winston, please try to understand,
Winston: No! You understand, I haven't ever judged you once. Not when you ate that ice cream cone that was bigger than your head. Not when you spilled a lot of that ice cream cone on your shirt and then picked it up with your hands and ate it anyways. Not when you ..
Me: Alright, Alright I know. But on the plus side, you get to eat fancier food for a bit?
Winston: Food huh? What kind of food.
Me: It is nice, I can even put it in this ball you can chase around and it'll dispense kibbles.
Winston: Hee jee look at it go! Oops a kibble yay!
Me: See, not so bad right?
Winston: Yum there's another one! Man this is fun. Leave me alone with my fancy snacks.
Me: So you aren't mad at me?
Winston: Wheeee! Snacks!
Me: I am going to call this one a win?
Winston: Chomp chomp nom nom.