I don't think that little kids can do what you are asking (the psychological response thing). Older kids and adults? Absolutely.
When I was in 4th grade, I went to an outdoor party and ate watermelon all afternoon long. That night after I came home, I started what was a 3 day throw-up fest. Of course, it wasn't the watermelon. I had a stomach virus and fever. But for the life of me, I wouldn't go near watermelon anymore as a kid. Smelling it even made me sick.
As a freshman in college, I got hives from eating strawberries. Four years later, I was able to eat them again without the hives, and I love them. (I am careful not to eat them too often or too many at one sitting - I don't want to trigger the allergy again!) However, my aversion to watermelon continued. When I had kids, they loved watermelon that my husband fed them, and it wasn't always possible to take it outside to feed them. I had to learn to manage the smell. I still won't eat it, but mostly because I have zero interest in putting it in my mouth. I know what it tastes like, even all these years later and I don't want it.
I do want those strawberries, though!
The point is, there is a big difference in what I am talking about and what is happening with your son. Food and additives CAN do what you are talking about, but it's physical, not psychological. Your son seems to have moved from an allergy to an intolerance, but given the way that he handles the intolerance, I would not feed those offending foods to him at all.
Dawn