Can I buy a train ticket for someone else?

We are in Japan from Thursday, our son (18yrs) will be arriving few days later at Narita but we will meet in Takayama. Can I purchase a train ticket (not a pass) plus seat reservation in green car for him to get from Narita to Takayama with my credit card? He does not have one.

Regular train tickets are not personalized. Anybody can by a ticket for anybody.

Will you give it to him personally?( then no problem there is no name on the ticket) Or do you somehow need to send it to him electronically? ( then I don�ft really know how to do that. Others might)

we will meet in Takayama.
Can I purchase a train ticket (not a pass) plus seat reservation in green car for him to get from Narita to Takayama with my credit card? He does not have one.

In that case, he cannot receive the tickets.

The same credit card has to be used
when you purchase tickets and when you receive them.

He does not have his own credit card.

Needless to say,
it's against the rule that he, pretending to be you, uses your credit card.

We are in Japan from Thursday, our son (18yrs) will be arriving few days later at Narita

How is your flight scheduled?

On the following conditions,
you may have a chance to send his tickets to him.
: You purchase his tickets during Thursday's daytime.
: He stays (not at a private lodging house but) at a hotel.
: He leaves Tokyo on Sunday or later.

Sorry, I did not catch the "we will meet" part.

People would typically (A) buy the physical ticket and send it to the user in advance by postal mail or (B) send the person money so that the person can buy the ticket by himself. In your case, it would be (B).

Train tickets can be bought before the date of usage. At Narita International Airport, there would be more JR personnel who can speak English. Your 18-year old can discuss it with a uniformed staff at Narita so that he would purchase the correct tickets, and if he needs extra help, he can call you through his mobile phone.

Hope you all have a great time in Japan!

If your son does not have a credit card, they can use cash. Presumably they are not arriving in the country with no means of buying a drink or snack.
If you buy using a credit card for him, you need to physically get it to him which would be difficult as you do not have a sending address he can receive it.