That is exactly how we got started with cruising - Disney mailed us a video. Be careful! That was 5 trips ago with a 6th already booked for next summer.
I wouldn't worry too much about sea sickness and the like. The ships are all stabalized and your son will be having too much fun to notice the rolling motion.
Have fun! Go for the 7 day trip if you can. The 3 and 4 day trips go by way too fast.