I vote for the mandolin. He could probabally find one cheap. I think Lark in the Morning sells a travel model thats tiny. Or perhaps a tenor guitar. Four strings and easier to learn.

I would also highly suggest a book called "The Backpackers Songbook". I got mine from Martin guitars for only $5.50. Great collection of songs, chord diagrams and some good first aid information included also. All in a book about 5 x 8 inches. Work checking out.