How to make him eat less?

Its a common myth that you can free feed (meaning leaving the food out all the time) all cats. The majority of cats can be free fed, but some can't. In that case, you will have to have meal times for all the cats, and the others will have to be on portions as well. Unfortunately that's the case when one over-eats. You can continue to give the non-problem cats as much as they want at meals (as long as they are a healthy weight), but you might need to limit the over-eaters portions. Consult your vet for quantities.