I wouldf suggest the Gordon West Tech study guide. Plain languaghe, common sense approach that is easily understood. So you help him with the math - its a good intro to simple algebra. It will be fun for bothy of you.
I have read through both the Gordon West, and ARRL. And have taught from the ARRL (May try Gordo's for the next class) Just make sure the book you get is for the new question pool effective July 1.