Since you're going in August, can you get to a carnival or fair between now and then to see how roller coasters go? When mine was 4 he loved them, but not all of them do. He also hated shows, still hates shows.
Keep in mind, he has no idea what is or is not at Disney. If he sees three awesome things he's going to be thrilled. He not going to be thrilled, however, if you run him ragged and drag him away from fun stuff. I remember spending about 90 minutes at the fountains in Epcot after an early dinner one night. That's it. We didn't go on any rides, see any characters, just ran in the fountains. I can also remember going on the Indy Cars over and over and over.
Have a plan for the day, but be totally willing to abandon it. If you leave after a few days with your son convinced it was the best time he ever had you've done it right!