Eshop code from Amazon Japan is quite cheap compared to buying locally.
I think the process for this would be moving my account over to Japan, which would also move his, redeeming the eshop code on his account , then switching region back to our local one.
Have I understood that correctly?
Then I have another warning for you. As long as an account has a positive balance, you cannot change its region. This is especially difficult with gift cards. So, as advised in another comment, it is better to have a separate account for this purpose.
Thanks again, but that’s still not quite the scenario I’m describing. This isn’t a code for a balance, its a code for a game. So there will be no issues on it being positive balance or not. Also while it isn’t possible to do it on your own while having a balance, that are plenty of reports online of Nintendo doing it via support.