IF the vet says he could do with losing some, then take their advice, they have seen him and know his body shape. As Charlotte says, wet food can help with this, and Dental foods can have more fat in them - I wouldn't go down the route of prescription diet food though, the ingredients are poor, and I didn't see any success with R/D. Increasing play and exercise will help - you could either throw his food piece by piece or hide it to encourage him to 'hunt'. As he isn't that young a cat, I would do things now, as there are health risks if he gets more overweight - but cats are normally heavier in the winter though, so it might be an idea to get him weighed again in the spring/summer.