Diet food suggestions

[Ffs2020]

Dcat has put on quite a lot of weight. He's 7, neutered, indoor/outdoor. He does not hunt. At all.

He usually eats royal canin weight management and AVA senior wet food. He doesn't usually get to finish his meals as my youngest dcat has a thing for everyone else's food. There's no medical reason for the gain, and I can't work it out. He isn't eating enough to explain it.

Anyone have any suggestions for a diet food to get his weight down? Also, he's very very fussy.

Is he being fed elsewhere? Seems likely if he's an outdoors cat.

I wondered that, but I don't think so. Our only neighbours have a slightly aggressive dog and he's a bit of a coward, if I'm honest, so I don't think he'd risk it even for food. He's also not the biggest fan of people food either.

Don't tell him its diet food. He'll be highly offended and refuse to eat it.