I hope I can teach him "Theory".
If the student ends up being serious about this then I agree that teaching him theory will benefit him ten fold down the road.
Yes, I agree. Theory gives the tools to go on and keep growing, Rudge hasn't explained much of the situation as yet, but I'm sure he will.

First off we don't want to turn the student off, who if they've gone to all the trouble to arrange to have lessons from Rudge must be keen to learn how to play the drums... Exciting .
Should I start with music theory.. or drum technique?
I think you'll need to assess your student by letting him/her play them while you watch, then assign tasks, then figure a study plan of Theory, and Performance.