Sodium benzoate intolerant?

For the longest time, my toddler was having unexplainable rashes. The nurse practitioner at his pediatrician's suggested additives may be the culprit, specifically sodium nitrite and nitrate. Sure enough, when we started paying attention to these ingredients, we found a connection to his rashes.

Soon after, I found online a list of preservatives people suspect to cause adverse symptoms. One ingredient listed, sodium benzoate, was in a certain medication my son had an allergic reaction to. Since then, I have been checking the labels on everything I buy, and there really seems to be a connection. But his pediatrician says she doesn't think he's allergic to it since it's in so many foods. Except since I've been refraining from giving him anything with SB in it, he's been rash free. Does anyone have any experience with this?

Also, I read that "allergies" to preservatives aren't true allergies, but intolerances (like lactose int.). What kind of specialist would we see for this?

He also seems to have reactions to the red dye cochineal/carmine, and like I mentioned, sodium nitrates and nitrites, which are in processed meats. None of these ingredients have shown up together on labels. All I know is if he accidentally eats anything with these or sodium benzoate on the label, he breaks out in a rash.

I think you are right that most preservatives/additives aren't allergies but intolerances. I also agree that you've identified what is causing his rash. I personally know that presevatives/additives can cause reactions because I can't drink some chocolate milk because certain brands cause me to break out in hives. I am not allergic to chocolate or milk but when the add all the other "stuff" it produces a nasty reaction.
If you want more guidance I would recommend a nutritionist or a naturopathic/alternative medicine doctor. I think you've done the hard part though which is discovering the culprit for the rash. Congratulations on the fine detective work!