Figuring out food trial fails can be very confusing! I would food test one food at a time. Do you know for sure it was the oatmeal or was it the peaches? Has he had peaches without oatmeal?
What was the ingredients of the puffs? Just rice?
A couple of things I learned over the last 2 and a half years:
1. Breastfeeding my son for a long time may have saved his life.
2. It can take awhile to develop a food reaction- my son happily ate rice cereal for an entire month before he had a severe FPIES reaction out of the blue- vomit to shock.
3. If you take a "break" from a food and reintroduce it, he can have a FPIES reaction. I try to keep foods in constant rotation. But, even then, we still have fails from foods he has eaten for awhile.
4. HOW the food is prepared and even brand can also make a difference. Though we have a few exceptions (avocado), my son does best with highly processed organic bought food or, when prepared, boiled or baked foods. For some wierd reason, my son can eat highly processed Gerber yogurt bites even though he had mild-moderate FPIES reactions to both whole milk and yogurt (trialed twice). He failed corn cereal, but he can eat certain corn-derived preservatives.
5. Read about the common triggers and avoid/delay introducing them. My son has failed most grains- check out quinoa. (For example, Quinoa Queen has a cereal product that has been an amazing new food for my son.)
Hope these tips help!