Originally Posted by JoeWPgh
I would sit him down at a keyboard and just go through the key of C for an hour or so. It's not asking a lot if all he has to deal with are the white keys, and it covers a lot of the basic theory he's not "getting".
+1, I was about to say that. He can even see what modes are made of that way (e.g. D Dorian is D to D on the white keys). Also, tell him that in the major and minor scales, as well as modes, that we use in Western harmony the notes are a whole-step apart except for two places where they are only a half-step apart (like in the major scale half-steps are between 3 and 4, and again between 7 and 8). He can also see that on the keyboard, as in the key of C where there are no black notes between E and F and between B and C. For any scale or mode all you have to do is move where the half-steps occur to change from one scale to another.
Studying the key of C on keyboard is the best way to learn theory. Once you get the hang of the concepts you can then begin to transpose.