yeah I would ask for advice from the vet. According to some of the basic weight calculators, a 12 pound dog with lower activity level should be getting 377 k/cals per day. I couldn't find the k/cals per cup of Nutro but for Before Grain Buffalo it was 383 k/cals so you are within the calorie limit according to that basic calculation. May be necessary to increase his exercise to long walks or fetch sessions or other activites that involve lots of running and play. It may be possible he has an underlying condition like hypothyroidism or cushings that are keeping him from losing any weight. What kind of dog is it? Super tiny corgi?