Training with treats can be tricky when your companion is already a little too heavy.
Have you tried any healthy, low-calorie treats, such as carrot sticks, apples (without the seeds), cabbage, green beans, broccoli, banana slices, or berries? Air-popped popcorn with no butter or salt can also make good treats. I have a friend who's dog goes wild for popcorn. You could also break plain cooked chicken into small pieces to use as training treats.