My heart just aches for you right now as I am recalling the painful journey it was in figuring out our daughters food "sensitivities." Her skin broke out between 2 and 3 months of age and as food was introduced her cheeks were red and blotchy all the time. That's when I started doing my own research.
My daughter is 3 now and I know that she is sensitive to over 25 food items. Many of them were diagnosed as an allergy, others came back negative on both skin tests and blood tests but still cause her to respond. One of those is strawberries! I mentioned it to one of the nurses from her Allergist and she said that there is a chemical in strawberries that is very similar to histamine. For some people, particularly highly sensitive people, when they eat strawberries the body recognises the "histamine" and says, "boy, there must be something we are attacking, we need to make more!" Thus the rashes.
After three years of dealing with this, my suggestion is ask your peditrician to refer you to a good allergist and have your son tested. It is a bit painful to watch, but much easier all around then having to live with the rashes and itching and heart ache. And, the sooner the better because, as others have mentioned, food allergies can be very, very serious...a whole lot more than a few pink cheeks!
Second, KEEP NURSING for as long as you can! It is very difficult to change your diet, but well worth it for your son. And, there may be wonderful benefits for you too; I lost a stubborn 30 lbs after changing my diet to accomodate my daughter!:-)
Finally, as others suggested, take the big 8 or 9 out of your own diet. Oatmeal has gluten in it, and that's likely the culprit.
Hang in there. I pray that it's a short journey toward answers for you and your family.