Welcome aboard, Suk Yee!
Awesome to hear that you're going to be sailing with Captain Mickey for the first time on the Disney Fantasy this year. You and your family will have such a blast.
First time cruisers on Disney tend to become repeat cruisers shortly after their first one... so don't be surprised if you find yourself checking out voyages for your next cruise while still onboard. They do offer a good discount if you book while still onboard :-).
The cast and crew onboard all the Disney ships pay very close attention to needs and instructions from all their guests. The galley crew and the dining teams are so used to seeing dietary restrictions and special instructions come through, and they pride themselves with ensuring top service that keeps guests' needs as their number priority. They really do make you feel extra special when you sail with them.
Diet instructions and allergies are a high priority and these are handled with the utmost care. I would suggest that you start by putting a special note about your son's food allergies on your current reservation. You can either call your travel agent or contact the Disney Cruise travel desk directly at 1-800-951-3532 to make sure that this important note is highlighted on your cruise reservation. When you meet your dining team on the 1st evening of your cruise, they would have already been advised of your special note, and they'll even review these with you before dinner. You are more than welcome to bring it up with your main server, as well as your head server. Both of them will be responsible for making sure that your instructions are followed. Your Dining team will be following you through your dining rotation each night - so at least, you won't have to keep repeating your instructions. Your main server will be taking your orders, and taking the food out to your table, so he/she will surely need to be advised of any special instructions.
Wishing you a magical cruise!
~ Dolly