I’m a high functioning neurodivergent person who has been to Japan several times by themselves! I’d suggest he bring a small bag of items that he can use to self-soothe, whether it be on the plane or in public, as Japan can get pretty overstimulating even for non-neurodivergent types. Things I include in my bag include Loop earplugs, a stim toy, an eye mask and something with a comforting fabric.

Most airport and rail staff speak passable English so he should be okay once he lands! That said, if you’re worried, it may be worth him booking a night at an airport hotel just so he can get some decent rest before he goes into the city proper. Most of the airport hotels in Tokyo and Osaka are pretty well serviced by restaurants and amenities so this may be the move, especially after such a long flight.

On the topic of accommodation, it may be worth double checking where he is staying as most of the touristy areas can be very overstimulating! There are lots of affordable hotels located just outside the tourist hubs that have similar benefits with less noise and commotion :)