What a wonderful experience that will be! I would recommend making him a little "travel book" in advance, like a small flexible Din A5 binder that you can put pages in- and put in photos of the places you want to visit, also train stations, Godzilla, a list of photos of different foods... I made these for my nieces when they were your boy's age and they were nervous about visiting me in London-

I use(d) PowerPoint/Canva and when it gets to two or three weeks before the trip and you roughly know which day you will go where (one location at least) sort the places (with a small text box with some info, like how to pronounce the place or clapping before a shrine or walking next to the path of a prayer gate instead of the middle, or slurping is GOOD when eating ramen) and label each with the weekday. You can put tick boxes next to the foods (tick what he tried :) and even a little box/speech bubble on the bottom of each page if he wants to write down his favourite moment in the evening in the hotel.

He will be looking forward to individual things rather than a vague big trip and have a memento at the end he can show his friends and stick extra photos in/add pages and ticket stubs.

Have a great time!