I used to do tip of my tounge to roof of my mouth. Tell him to relax his throat and bring the tip of the tounge to the tip of the reed.
To help with relatxation, he can litterally put a sock in the end of the neck in such a way that the resistance in nearly impossible to blow through. This is a clarinet trick, but seems apropriate for this. In theory, he'll have no choice but to relax just because of the pressure, and he'll play more relaxed without it becuase he exhaused the muscles that casue tension. I can't remember if the octave key should open right now...
As for actually tounging, I'd just have him tounge over and over again, making sure he pays attention to bringing the tip of his tounge to the tip of the reed. If he wants to do it without the sax, the motion can be praticed by bringing the tip of his tounge to where the gums and teeth separate, similar to trumpet articulation.
Both of these excercises are best done wothout the sax at first, just mouthpiece and neck (sax neck, that is
).
Thats how I overcame my bad tounging habit.