Cooper is a gorgeous boy!
We home seniors and Cooper is right on the cusp of seniorhood. I've learned from here that "senior" formulas are really not that great. I feed Fromm, a cup in the morning and a cup in the evening. To fill the tummy I use frozen or canned green beans (no salt). Fruit is also good for a snack but they do have sugars in them. Raw carrots are great along with a lot of other veggies. Broccoli may cause gas. NO mushrooms, onions, grapes or raisins, they are bad for dogs.
If you could up the exercise that would help too. Can you get a dog walked to come in mid day for extra exercise or take longer walks. After dark we play games in the house, fetch, find it, basic obedience.
Archie just dropped about 8 pounds but it did take a while, six months or more. Steady slow weight loss is better.
And don't let him guilt you out. Labs are very good at that.