Hi K.
I know it seems overwhelming. We just found out 4 weeks ago through a blood test that my 7 year old daughter is highly sensitive to wheat, eggs and cow's milk/dairy...talk about a limited diet! And her being 7 years old has been a challenge since she basically lived on pasta/olive oil, mac and cheese, and dairy in every form! I am just lucky that she happens to like fruits/veggies or she would have nothing left to eat. The good news for your nephew and my daughter is that these are sensitivities and not allergies. They are not dangerous and can actually have a good chance of being eliminated if the proper avoidance diet is followed for a long enough period of time.
The best bet is to introduce your nephew to a lot of fruits and veggies and "pure" foods that are not processed. Since your nephew is not allergic to wheat/gluten...he has more options than you may think. Grocery shopping should be done at stores where they sell items made from simple ingredients (ie bread made from flour, yeast, salt and water)...or offer labels/brands that advertise what the items are free of. Whole Foods offers many items an 18 month old might like in their gluten free area (many labels read NO DAIRY, CORN, SOY, GLUTEN, etc. right on the front). It is essential to read labels, but your sister will get used to it. Cooking from scratch is also a key to surviving this time. Corn is in way too many things that are prepackaged. And the cooking does not have to be complicated. For example, pancakes can easily be made from flour, baking powder, sugar, egg and rice milk (replacing regular milk)...and then they can be frozen with wax paper to separate them so that they can be easily available at all times. I have always been big on freezing, but it is now extremely helpful to always have something on hand in the freezer when there are sensitivities involved. And really the best way to address it is to offer foods that are free from corn/dairy naturally...fruits, veggies, fish, meats, good breads, sorbets, etc. Because imitation cheese (for example) is just plain nasty. And as far as fun kid-friendly food goes, Whole foods offers a huge selection (ie chicken nuggets) that are free from corn/dairy. Bell and Evans offers fantastic frozen chicken nuggets...just don't get the gluten free because it has corn (and unfortunately for us, egg too).
The most important thing to remember...and a lesson for all of us to take from this...is to not overdo any one main ingredient. Sensitivities can occur do to an overload on the body (ie having an ingredient every day/many times per day...like most kids).But when you can't have a certain food anymore, it is sometimes easy to start replacing that food with a new "overload" food. So try to keep variety in his diet. Also, when he reintroduces the food (after a minimum of 4 weeks but optimally between 60-90 days for best possible odds of beating the sensitivity), he should try just one of the foods one day (not overdoing it that day), and then none of that food the next. He should continue on this every other day routine with for about a week. If no 'symtoms' occur, then try the next reactive food in the same every other day fashion. And if he beats the sensitivity, just always keep 'moderation' in mind. I know that is my plan because if we are so lucky as to eliminate her sensitivities, I will do what I can to never have to do this again!
Lastly, I would be cautious about soy. Where corn used to be the most common food in almost all processed foods, I feel like soy is getting up there too. Read labels, you will be amazed. Also, soy's phytoestrogens could possibly disrupt a child's natural development process. The jury still seems to be out, so I err on the side of caution. Because my daughter's menu is so ridiculously limited, I do allow about 2 chocolate soy milk drink boxes per week, but I am nervous to go much beyond that.
Hope that some of my babbling will be helpful.
Best of luck!