Do you have any idea of how tall he is? If he's pretty tall, you could probably go with an adult size length. That probably won't help you a bit because scarf patterns run from the 50's all way up to the 70's, at least among my patterns.

Could you find someone about his height and try a scarf on that person? (I'm assuming this is a surprise gift.)

No surprise, but he told me yesterday that fingertip to fingertip is too long for him. I should have measured him! I can call him - thanks.

I think I remember hearing that finger tip to finger tip was equal to a person's height.

Jane2 here (Janey on knitting blogs)