I have a severe allergy to mushrooms and truffle so I totally understand the question because it is so unique. Here are my tips:
1) If you booked with a
travel agent make sure the allergy is noted on your reservation. If you booked on your own, contact
Disney Cruise Line directly (sorry probably a long wait) and have them note it on your reservation.
2) On the first day you board they will direct you to the buffet or quick service options. I just ask for the head server, they will come out ask about the allergy, then take his order of whatever he wants and will make it for him in a sterile area of the kitchen. Any time I go to the buffet I do this and my food is always nice and fresh and just what I want. Normally more than I want too;-) If you want more they will always make more too.
3) FYI...back in the day they used to allow you to take a tour of the kitchen gally. They showed us the area where all allergy meals are prepared. I got really excited about it and then noticed a bright pink piece of paper with my name and my allergies and options to suggest. My husband prefers Disney because of how serious and personal they take it.
4) Your head server and assistant server should already know about the allergy before you board but will talk to you about it the first night you dine with them. This is another reason to make sure you list the allergy on your reservation. Let them know your concerns and what you would like as your experience and they will make sure you are comfortable and happy. Even days I did not pre-order they have always been able to help me.
5) Self serve stations/quick food. All these areas have ingredient sheets or you can always ask the cast members to check what the ingredients are. After a few times they will even remember and help.
6) As others have said, at the end of your meal they will bring out the next nights menu for you to look at and choose what you would like to order for tomorrow night. Sometimes you will arrive the next day and want something else. That is okay. You might have to wait just a little longer. However, when I would sit down I would always ask...what did I order??
7) Pack extra zyrtec, stomach medicine that helps, and anti-diarrhea medication. They will also give or deliver crackers, sprite, etc if his stomach gets upset. You can ask room service to bring it to you.
Hope that helps and have a wonderful vacation.