My Moose (the one pictured in my avatar) was almost 22 lbs when I got him. Now two years later, he's down to 16.5 lbs. I have four other cats, so it wasn't easy. Separate feeding areas, food, treats, no treats, etc., was either a lot of trouble or felt too much like punishment for one of the parties involved. So, I increased his level of activity. Turns out this big guy really loved to play. I engaged him in different levels of play several times a day. So, why not give exercise a try? You'll both have fun, and it will be much less stressful.
Want to add that Carmello is a handsome boy!! Keep us posted on his progress.