Tell him you have a slight crush on him and would like to know him better as a friend. Honesty is the best policy.
He'll do one of three things: 1) reject you completely 2) reject you in a polite and indirect manner which is the customary way to go about rejection in Japan, or 3) He'll be delighted and reciprocate your feelings.
Life is too short to *wait* for the perfect moment, so you might as well just go for it and if he opts of number one, you'll know he wasn't a very nice guy to begin with and it wasn't meant to be. The second option wouldn't be bad either; it'll let you know you need to move on and not waste time.