I would just suggest spending a little more play time per day with him (cat owner for 15 years) and if you can, feed him separately from the others. Feed your other (4?) cats first while keeping the overweight one inside or in a different area until they are done, and then feed your special guy. If you can, ask a vet about a good diet cat food to get him on, and go for that, if you can't then just feed smaller amounts of food, or buy some diet brand cat food.
That's the best advice I can give ya, extra play time is important, especially if your cat is getting older and doesn't like to play as much, try different activities, string, feathers, ribbon, the little red laser light of doom, any thing that might get his attention. My Shadow, before she passed away last year, loved playing with those little felt mice things xD
5 bettas: Шостакович Shostakovich, Dimitri, Blood, Fancy Pants & Miss Priss; 1 cat: Rajah