Our group has rotated DM chores a few times so that each of us can take a break from running the show to recharge our batteries. One of the fellas was a bit heavy-handed when at the controls, and we found ways to subtly express displeasure during those situations.
As several people herein have suggested, a direct, honest, respectful discussion about your concerns is best. If your friend is the type of person who will almost certainly react poorly to this method of approach, then try to put a bug in his ear by making background remarks which praise previous campaign highlights handled in the manner you most enjoyed. If he hears enough commentary about things that went over well in the past, he might shift gears a bit. Admittedly this is less useful than open discussion, but there are some people who just cannot bear criticism without getting their undies in a twist. I've found directed compliments helpful in these instances.
One question -- Are you the only group member perturbed enough to speak out regarding recent campaign events? If not, you might want to consider spending a few minutes before the next session (or afterwards) discussing groupwide concerns and frustration. Not just about the DMing, but PC interaction, etc. If other topics are covered the DM won't feel singled out and attacked.
