Given the distance involved, it would be easier to send CDs in the mail than to get the MP3 player back to add new files. You would not be able to teach him in person to use the MP3 player and most people can work a CD player without too much difficulty.

Yes, it looks like you did find a reliable source for audiocassettes. Unless you are planning to talk for over an hour with no one around to hear you, maybe you could get 60 minute cassettes instead of 90? The longer the duration of the cassette, the thinner the plastic film tends to be. The thinner it is, the more likely it is to snag or break with multiple listenings.

Having a cable to connect your computer to a cassette recorder / player means that you could probably record on your computer and edit it and then have it play back to record to the cassette later. Just watch for the 30 minute or 45 minute mark when you have to flip the cassette to keep going.