Welcome aboard, Breno!
I'm happy to tell you that Disney Cruise Line does their absolute best to ensure wonderful dining experiences for each and every guest, no matter their preferences or special dietary needs. They are very accustomed to special needs in the restaurants, and can certainly work with you on this.
I'd recommend calling Disney Cruise Line, or your travel agent, and noting your son's dietary information on your reservation. This way, your servers will have this information before you arrive. The first evening, your head server will come to your table to discuss options for your son, and offer you options for the next evening as well, should it need to be prepared ahead of time.
Disney Cruise Line is the only cruise line to offer
rotational dining. You will be assigned one of the three main dining restaurants each night of your cruise, and your servers will follow you, ensuring that they learn your needs and preferences and you don't have to re-explain each night. You'll get to know them and they'll get to know you.
Give it a try the first evening. The experiences in the dining restaurants are part of the magic of sailing with Disney, and you will be happy that your son can experience them with you. I hope this helps you plan, Breno! Let me know if I can help you as you prepare to set sail!