If he wants to focus on a single element, I'd suggest sorcerer or wizard and then going into the Elemental savant prestige class from Complete Arcane. Their whole schtick is about focusing on a certain element. For spells just look for ones with the right elemental subtype (i.e. [fire] or [air]). He can get creative too. For example a lot of force spells could be flavored as wind. And other things, like Airbenders being able to run really fast can be mimicked by something like haste or expeditious retreat. And fly would be a must of course.
If he wants to be able to control multiple elements, it gets a bit trickier. Your best bet there might be to just go sorcerer or wizard and take various elemental spells.
Or you could to Jade Phoenix Mage. That would give him martial arts (swordsage maneuvers), as well as fire-themed desert wind stuff, and your sorc/wizard spells would pick up the rest of the elements.