I purchased Dramamine for children and kept that on hand. But did not need to use it. My six year (Last year he took his first cruise at five) was fine. We had an aft cabin and ate in the Monet Restaurant (which was in the aft of the ship) and the motion did not affect him at all. If you are concerned, give him the dramamine before you get on the ship. Also, you might want to speak with your pediatrician - he or she may give an prescription for the patch - I don't know if they are available for such young children.
I took my children last year and brought dramamine with me. It was my first cruise and I get seasick in a boat (such as deep sea fishing or a catamaran), yet on a five day cruise I only took one pill. None of the four kids needed any.
I started both kids on Dramamine 1 day before we sailed and gave them it every day....Pediatrician suggested this since both my kids get car sick...I did this...and even through 2 days of ROUGH waters neither of them got ill...
We have always used Bonine (over the counter). It works like dramamine but without any drowsiness. Take 1 every 24 hours. It's probably a good idea to start taking it a day ahead of time so it gets into your system. We have a 9 year old susceptible to motion sickness and for the last few years we have given her half of a pill and she has never gotten seasick.