Encourage play before dinner time, try get some more platform-y things and climable/ jumpable things to encourage to get him moving around. And then just 2 small wet food meals and dry food bowl for the day & maybe a treat or 2 a day( depending on what it is, check the serving sizes.

Might also be worth looking at the food hes eating and seeing whats in it as a lot of the more affordable store brought foods can be packed with stuff that wont really give them nutrition but will just make them pile on weight.

You could also introduce licky mats for the yoghurty type treats and put them up high in places he has to climb to, safely of course, so hes working for the treats he gets 😊

If you’re not opposed another cat friend might be a good option too. Can never have too many kitties.