does not necessarily have to be an allergy, because if it were it would happen everytime he ate a tomato. I think its more like a reaction and could have been because he ate tomatoes sin access at a certain time and then consecutive eating has keep the reaction going. IYKWIM.
I would stay clear for a few weeks and then slowly introde back and see,
DS loves blueberries sand gnats to eta a whole box - is too old for me to try and hide it from him, but gets a rash when he eats too many, so we are avoiding them for this season..