E.: I am not famililar with the FEingold diet, but at an early age I realized my son was having allergies to my breast milk (what I was eating) and the Drs back then said impossible. (He is 9 now.) So, I took him for rast testing (which I found out about by researching online) and found that he was very allergic to many things ---PEANUT, oat, wheat, dairy, egg, etc.....he has grown through some, and they are now what I call food sensitivities...as long as we somewhat rotate through foods --except for Peanut. (Although I do not monitor it now as much as I used to.)
All that said by way of suggesting strongly that you listen to your own instincts.
In addition, I clearly identified along the way (mostly by behavioral responses) that he had sensitivities to: Red 40, phenoylyn (sp), aspartame, and titanium dioxide. As we observed, it became clearly obvious by his behavior (soon after ingesting). He became a different kid when ingesting these items.. Soon he could also beging to identify the feeling "inside" of him if he had eaten something. Even to this day, I can see him and say, what did you just eat?????!!!!!!!!!!!!!!! And then, we both say, ahhhh...
He is also slightly gifted, not off the charts. I just wanted to mention because some of these behaviors and sensitivities seem to go hand in hand with gifted children. Usually you cannot test before age 4, but at age 4 we tested, and it was somewhat of a relief to understand that this also contributed to some behavior issues.
I hope this has helped. At the very least, by suggesting strongly that you listen to your instinct. There is more to our story, but in the interest of space, I have omitted.
Best of luck to you. Know that you know your child better than anyone else, and that NOTHING can replace a mother's instinct. If you move forward with that intention, you will surely take the right steps for your child. And, as much as possible, relax and understand that you are doing the best you can. Namaste.