My son just turned 5, and we've had some similar experiences for the past year- more aggression, more "attitude", etc. We have talked to our son's teachers and read books and articles, and came to the conclusion that boys who don't know how to deal with their anger properly tend to act out their anger. The key is to find a way for your son to deal with his anger, that doesn't involve physical aggression. We made up a cute little rhyme about being mad that we yell while we stomp our feet on the ground, and it tends to lighten the mood when my son gets angry. The physical aggression has been minimized tremendously. The other suggestion someone gave us was to teach our son to take deep breaths in and out when he's upset, to calm down. That has helped too.
Wish you luck, but know you're not alone.