Would he be better off with typing out text, or adding images instead of drawing them? Or do you think the handwritten text makes the videos seem more personal?
A really great example of a superb mathematics textbook by a top mathematician, all with hand-drawn diagrams, is Geometry: Euclid and Beyond by Robin Hartshorne.
http://www.amazon.com/Geometry-Euclid-Beyond-Robin-Hartshorn...
The author explains his rationale for having hand-drawn rather than computer-generated geometry diagrams, discusses why he wrote the book as he did. I love this book as a remarkably accessible book on some very deep mathematics, with wonderfully challenging problems. I think Khan Academy video lectures work for some of the same reasons that Hartshorne's approach in Euclid: Geometry and Beyond works, and I hope that Salman Khan and his collaborators eventually get to advanced geometry like that in Hartshorne's book in the Khan Academy course list.
Showing concepts in animations while explaining them can be helpful, but ultimately the students are learning how to solve problems. Watching the strokes of his pen gives you a fantastic view of how the problems are actually solved on pen and paper.