how long has it been since he has had anything that he reacts too??
when mine were tiny and reacting, I had to 'check' the reaction every few months to see if they had grown out of it at all, or if they were beginning to develope some resistance to it......ie.....after a while, a biscuit wouldn't cause a reaction, yet something with higher milk content would.
Because I tried every so often tho, they got to know that their body didn;t like sertain foods, making it easier for them to understand when I said they weren't allowed to eat somehting IYGWIM.
At 2, mine were certtainly able to handle trace amounts of milk, as they weren't allergic, just intolerant.
I, on the other hand, am allergic, so trace amounts make me VERY poorly.....but then I am an adult and so understand this....obviously.
It IS much harder with tinies, but sometimes, the only way is for them to be able to liken it to a certain feeling. if he has been ill recently you could refer to that feeling, as to feed him said food just to 'show' him could be classed as very very mean!