Battle of the bulge, doggy style

You remember my Min Pin, Sparky, who I put on a diet since he ate an entire french bread loaf. He had been doing well until 2 weeks ago when he seems to have found another source of food. I found him raiding one of our other dog's food dish.  Foxy, the other dog, has decided she doesn't like the dry dog food and has been leaving most of it in her dish. This is to the delight of Sparky who happily eats the left overs.

This is a dog that is starting to waddle when he walks. He use to run all over the place but now he has no energy because of his added weight. I am thinking about putting him on our treadmill at home just to get him moving again. I wonder if they make diet pills for dogs? Other than that I could always get him a muzzle just to keep him from eating the other dog's food.