The Kong package will give the weight range. Depending on chewing style, you could probably go with a small or a medium. As for the weight gain, you don't need a different food to get him to lose weight. 3 pounds on 15 pound dog is a 20% weight gain, which is a big deal in my opinion. I would just cut his food back up to 25%. You should easily be able to feel a dog's ribs with the pressure of petting. It should feel like the back of your hand.
Hope he loves his birthday toy!
First time Chihuahua owner with many years of dog experience!