I had a similar experience with one of my close friends recently. We were going to go out to eat, and he insisted on going to one of our favourite places we used to visit, before we figured out what was making me sick. He proceeded to mock me about it, accuse me of exaggerating, and insist there was something on the menu I could eat, because it was "ridiculous to believe that anywhere had nothing for me to eat". I ended up telling him he was being insensitive, and leaving.
I let him cool down and after a few days he came back to talk to me about it and ask questions about how serious it was, and I realized I needed to be more specific in giving actual examples that relate to me, not general examples, in order for him to put how it effects me into perspective.