As a 53yo, whose collegiate days are long behind her and who has never seen the need for getting high on anything other than life, I can tell you that his following goes far beyond the college crowd.
The reason is that he's different from almost every other commercial band out there. He does things musically that noone else is willing to do. Turn on mainstream radio and every song/singer is just a variation of the same thing. Dave does things with instrumental arrangement that is unique today. He's not afraid to experiment with sound and arrangement. He thinks outside the box. I'll give you that his vocals and lyrics leave something to be desired, but the use of varied instruments and the combo of different sounds is what has earned him such a huge following.