Spock is gorgeous. Putting a cat on a diet takes will power. It may help to spend time with him getting him running around playing, more toys and climbing trees and things that encourage Midnight Crazies may help.
Ari is twelve. His Home Day was July 6, 2000 - he was six weeks old and about 3/4 fur. He's 15lb but not fat, he's got legs like a panther - thick and muscled - and while his skin hangs pretty loose like he's a fat cat or ex-fat cat, he lays completely flat and when I pet him, he's got a spine and I can feel ribs without them being bony-too-thin.
He's just a well muscled big cat. I wouldn't be surprised if Spock would be closer to fit rather than fat if he dropped two pounds and put on muscle.
Ari turned into a Rocket Kitten on day two. He's never stopped. He treats my bed as a trampoline. He'll yell and jump up on it and chase his tail jumping up to swat at his tail, spin in the air and land to do it again. I wish I could get a video of the Flying Tail Chase thing he does. When he does that I get out the feather wand and get him pouncing at it off the bed and across the room.
Things that you can hold out or throw at arm's reach so you're not running around trying to get the cat to run work well for cat exercise. The trick is to catch them at the moment they're bored and looking for something to do, then enthusiastically play with them and appreciate every funny or cool move they make. He knows I watch his Flying Tail Chase thing every time and will do it to get attention if he thinks I'm too lost in the computer.
Here's a photo - got to put in a photo of the handsome Ari!
I hope that works, it's a Facebook photo.
Okay, let's put it in as a link...
Ari Picture. That's my fuzz buddy curled up on his seat on our Two-Level Cat-and-Man Armchair. It's a great chair designed with a high seat for cat and lower seat for human, puts us at eye level with each other in great padded comfort.