I think that ideally a dog uses his partially open jaws to bounce the ball, like he tried to catch but the ball is too big to go in his mouth? If he doesn't get the idea straight away perhaps throw him a ball he can catch then a ball he can't - so he is in catch mode? I use a slightly deflated soccer ball but with your little Hunter maybe you could find something softer and much lighter - something as hard and heavy as a tennis ball might hurt his nose/jaws. I'd love to know how you get on as I've never seen anyone else teach this trick.
Zac is very focussed on my hand and body signals too. I am trying to teach myself to keep still while giving commands, Zac is very good at learning signals that I didn't mean to teach - like a that step towards him when he is sitting means "sit pretty" I know I need more training than the dog!