Go down to your local bike shop and look about getting a 20" wheeled unicycle.
Something like that shouldn't cost much more than $100 and it'll be good enough to learn on. After he's beaten that into the ground, he'll have more of an idea of the type of riding he likes, and you can go from there.
www.unicycle.com also has a ridiculous amount of unicycles to choose from, if you're so inclined. I recommend the bike shop route, though.
