I have a similar problem to Gio. My guy Scooter has been packing on the pounds, especially since I moved in with my sister.
My sister had 2 older cats, one of whom has pretty much lost her appetite. The vet advised my sister to have food out at all times for Ebie, in case she temporarily gets her appetite back. This has worked out fairly well for Ebie, she has gained about 1/2 a pound, but Scooter has been abusing this. He gained 1 1/2 pounds!