the XR, and other iPhones use a nano-SIM (and not a Micro-SIM, they are different sizes). If you send him a nano SIM tied to an active plan, it will work on the iPhone XR. Something I would definitely make sure is if the iPhone is unlocked or not as I am not sure how Japanese telecom policies work.
Personally, I run an unlocked US iPhone (purchased through Apple) on T-Mobile out on my trips to Asia with no issues. T-mobile goes into roaming while overseas, but I usually find a local carrier and use their network either via nano-SIM (physical) or eSIM.
I would recommend having your nephew check with his carrier and you inquiring T-mobile just to be sure, but I have had no issues so far. As the nano-SIM isn't tied to one device, another method might be for you to activate a SIM (via your iPhone) then ship it to your nephew.