If you get a tape recorder / player you can also get a cable to connect your tape player to your computer's sound card and record from the cassettes to software such as Audacity. Then you can save the songs as MP3 files.

Be sure you can get enough blank cassettes, or you will have to have him give them back and record over them. I don't see blank cassettes in stores much any more.

Maybe you could just get him a cheap portable CD player or MP3 player and teach him to use it. It would be easier.