Hi Stacey! I'm so happy you stopped by the Disney Parks Moms Panel.
You are correct, children under the age of 3 are special Guests of Mickey Mouse and do not need the Disney Dining Plan. However, your situation with allergies can be tricky. If you personally order food that will fit his dietary restriction, he is more than welcome to share off of your plate. At sit down restaurants, if you need to order him something extra, you will need to pay for that out of pocket. I think, your best option is to dine at all-you-care-to-enjoy restaurants. Buffets are going to fit your needs the best. When you make your dining reservation, be sure to note on your account you have a person who will be dining with food allergies and then remind the restaurant again when you check in. Most of the time, the Chef will personally come out and walk the buffet with you pointing out all of the options that are available to him with his food allergies. If you still don't think that is going to work out for your family, you can bring in outside food to a restaurant when someone has a food allergy. So you may be able to pick up something somewhere else for him and then bring it with you to the restaurant. The
special dietary request page of the Walt Disney World website also has some great information regarding dining at Disney with food allergies.
I hope this helps answer your question. If I can help you with anything else, please come back and ask.