Hello,
I had the same problem with my maltese. I had him neutered and gained all kinds of weight. He went from 7 lbs to about 14 lbs. AHHHH! My vet suggested that I make his food, it's a pain but it worked. Baked chicken with potatoes. I also walked him for about 20 minutes everynight. I got him down to 9 lbs and he has stayed there. Good luck!