From my understanding, as long as the what is sold is immaterial (e.g., membership, software) and doesn't need to be sent to a specific address, you don't need to register as doing business in the country where the good is sold.

So in this case, I think that he might not need to register for doing business in japan if he's not selling any tangible thing...

I'm not so sure though since I don't know how it is in japan.