last trip to disney was in may, my dayghter was 15 months. my daughter is allergic to milk,egg, nuts and soy. i researched before going to wdw. if you go through the disney web site there is a link for food allergies. they will send you a list of food that is ok to eat and where to find them. when you make ADR tell them so that they can make a note on your reservation. once there, tell the waiter/waitrress again of any food allergies. the chef will come and make a special meal.
for your grandson's food allergy i would not worry about it too much, he will be well taken care of. my daughter is also allergic to soy. soy is a tricky one. they put it everywhere now, candy,cookie,bread... i went to the pinocchio house in mk and when i talked to the chef, all he proposed was a platter with 4 pieces of turkey cold meat with carrot sticks and i was charged $7.50. no where on site could i find rice milk. in dowtown disney they gave her plain salmon,about the size of 1/2 deck of card with sushi rice size of a shooter. they charged me $7.50. needless to say my daughter even though she was a baby was still hungry. i had to ask for more rice. i was fed up so i went to the grocery and bought pita bread and ham and made her a sandwich for the next day. at least i knew she would not starve.
in summary, for most food allergies, you will have no proble, but i would bring a few snacks or sandwich to make them wait until the food is served