My little Tatum is too chubby too
One thing that you can do is to feed your baby the amount of food that a dog should eat that weighs what he is SUPPOSED to weigh. Just for example - if he weighs 7 lbs now, don't feed him what a 7 lb dog should eat. Feed him the amount that a 4 or 5 lb dog should get if that is what he is supposed to weigh. I would feed him twice a day. I would not recommend free feeding if he is over weight.
Also, as the other 2 posters have already mentioned, Green Beans are GREAT as they are good for them and they are fillers so it will help him to actually feel full and not hungry since he will be getting a lesser amount of food. I give these to Tatum and she absolutely loves them. She gets Organic Green Beans with NO Salt added.
Another thing is exercise, exercise, exercise! Take him for at least 2 walks a day...start off slow, about 10 or 15 minute walks each until he gets used to it, and then you can increase the length of time that you walk him for. If he isn't used to being walked, he will probably tire very easily.
Another thing that you can easily do at home to get him moving and give him exercise is just to play with him. Play fetch and tug of war - just sitting down on the floor with him and playing with him actively will give him exercise.