There aren't any major Japanese holidays going on when he'll be in town. July = fucking hot and humid in most of Japan though, so prepare for that.
There have been a few 'going to Japan, halp' threads in GD, quick search turned up the last one on the topic:
http://www.bluegartr.com/forum/showthread.php?t=90235
It might also help if you listed specific interests. If you're not much of an anime/manga nerd Akihabara might not be that interesting. What you're planning on buying and bringing back will probably dictate where you spend most of your time in Tokyo.