The people commenting with " get over yourself" clearly don't have to deal with an intolerant child on a day to day basis. Count yourself lucky
I was that child (actually with an allergy rather than intolerance, but still). I grew up fully accepting that I couldn't have treats like chocolate, but that it was normal for other children to, and this would not have upset me in the slightest (though my parents would probably have made sure I had something nice instead).
I was far more embarrassed when people would realise, get awkward and then either remove the chocolate from everyone, or desperately try to find something else. I remember vividly at the age of 5 a friend's mum made her daughter find me a teddy from her toybox- I knew what was happening although she tried to pretend it was a new one she'd bought for me, and I was mortified.
You need to teach your son that he can't eat things like that, but that he can still get involved. Don't make him feel like an alien - he will not thank you for it. No kid wants to feel like a weirdo. I was very capable from a very young age of not eating anything without checking first, until I got old enough to learn what was ok myself.
Enable your son, don't wrap him in cotton wool and get rage over little things like this.