I'd go with an unlocked tri or quad band phone, then just pick up a prepaid sim card where ever in the world he happens to be travelling. It's a lot cheaper than having to pay for a "world plan" and roaming charges. The only downside is that your number will change with each prepaid sim from each country. I've done this.
Depending on the phone model and the connection speed of the country that he's in, he could also opt for prepaid data rather than voice, and then run voip over the phone. And that way he'd get to keep one number. I've not done this, it's only in my theoretical realm right now, though I'd love to try it out on my next trip abroad.