Im not sure about that particular drug but if you talk with his doctor he may be able to get a longer supply for the trip. I get most of my meds via mail order and I'm thinking maybe the doctor could send a scrip to a mail order pharmacy. I know mine is called Express Scrip. Its one the military uses so I'm thinking they may ship worldwide. As a retired military I'm pretty sure a moder country like Japan probably may even be able to get the medication if your son contacted maybe a local pharmacy in country? As for the echo I'm pretty sure even that could be done in Japan also. I know for instances mayo Clinic has provided medical services to foreign patients, so it seems possible for anyone to get treatment overseas.
As a side note I remember asking a travel service in Israel if being a Heart Transplant patient if it would be a problem to come to Israel on vacation if something happen could I get treatment in Israel and the answer was a Yes and in my case, Israel does transplants in TelAvive so no problem. I'm guessing the same is possible in a country like Japan also. Please let us know what you find out? it may help others deciding on travel overseas.

I found this article about this medication and availibleaty is mentioned in Japan here is the web site. https://pharmaphorum.com/news/bms-buys-asian-rights-key-growth-drug-camzyos