If that doesn't make him want to be trained nothing will. But he is going to look funny on that by the time he is 16, he just won't want to use the grown up one. You need to make a big one that will fit over the grown up one.
Great idea and as slow as my little grandson is taking to the training, I still have time to make him one. Wet, poop-ie pullups don't seem to bother him a bit!