I'd say with about 99% certainty that Dave Matthews is an ISFP.
Explanation -
I - He would spend hours alone as a child, just thinking, and often expressed his inner struggles through music. He was also naturally afraid of performing, but excelled at it obviously with time.
S - Fits the mold of Sensing perfectly, as he is very much focused on the here and now and is known for his sensate expression in performances.
F - Clearly a feeler, Dave's lyrics and personality indicate that he feels intense emotions on the inside. He is driven to find the pure and ideal.
P - Definitely a perceiver, as he does not prefer a structured environment at all and did not excel in the education system.
ISFP Profile
Read the top section of that, and it is a perfect explanation of Dave's personality.
Finally, I am an ISFP myself and have always closely identified with Dave's music in a profound way.