I looked in one of my books and I think this is going to take you forever. If you have the patience it looks like you need to let one leader grow taller than the remaining ball. Just let it grow - let it grow - and it will eventually branch out and you can prune this into a ball shape. It depends on your boxwood variety how fast it will grow. I have compact boxwood and it only grows an inch or so each year. You might have better luck starting with a new boxwood and cutting the middle out and shaping and upper and a lower ball. good luck