If you're using him now... I'd 5-star. Who knows if you'll be on him at all when they add his breakthrough. Meanwhile, you know there's real value to be had now, if you're on him. That's actually my biggest problem with collecting ninjas - too often you lose power you could always use versus getting a ninja that you don't stay on long enough for it to be worth that loss. Here, if you wait then you know you're sacrificing power so you better be quite certain you'd get the value back in the future.

Looks like they're actually adding skillbooks tomorrow, so you can be happy about that. Just be prepared to be angry in a month or so when they give the books to everyone via login rewards. :P