You may want to speak to a naturopath about trying nutritional therapy. It's not just eliminating things in his diet, but getting recommendations from the naturopath on what you can give him (likely in excess) that will regenerate the cells to heal.
The body does not choose what to heal first. It heals what is needed, when it is nourished.
The typical lunchable or peanut butter and jelly sandwich does not nourish the cells in the body. High nutrient content raw veggies and fruits however, do. There are ways to get picky eaters to eat these things, you just have to be creative.
For example, our "smoothies" are packed full of red cabbage, spinach and kale. It's simple: 6 ice cubes, 1/2 cup frozen berries, rinsed leafy vegetables, 1 scoop chocolate protein powder, and cold water. Blend till smooth.
When I use blueberries the smoothie is as purple as someting you get at Jamba. It's sweet, creamy, cold, and they LOVE it!
The fresher it is, the more nutrient dense and better for the body. Cooking foods immediately reduces nutrient and protein content by 50%. So the more fresh concoctions you can offer your family, the better.
If you have a juicer, you can experiment with different veggie and fruit combinations to see what works well together. And then you can use the pulp to make muffins - a little deceptively delicious. It's a great way to get kids vegetable fiber without them even knowing it.
Spinach can be added to just about anything. Pineapple is the best fruit to aid in digestion. Avocado spread on bread is a great substitute for mayo, or try hummus.
Try to stay away from hydrogenated oils (basically anything made from corn), white sugar, corn syrup, food dyes, preservatives. If there are more than 6 ingredients on a packaged food, or if you don't know what they actually are, you should not be eating it or feeding it to your kids. Is this always feasible? No, but you can control what he eats in your own home.
Good luck !