fat cat help?

Ca

I know this isn't something usually on this app, but I'm sure someone on here has a cat and some input. I have three cats. I changed their food so they have the healthy indoor weight formula, and I monitor how much they eat. But one of my three is still really fat, and he gets more exercise than the other two, so I am confused. How do I help him lose some weight? I don't want to be one of those pet parents who lets the health of their animals become bad due to being overweight. Help? Please?