The kind of gluten that people usually react to are only in wheat, barley and rye (and usually oats by cross contamination). So the rice cereal would be something other than gluten. Have you tried it since? And it would be unusual for your lo to be ok with the biscuit for three days and not the fourth. I would be more inclined to chalk it up to gagging. Possibly a stomach bug? Does he have any other symptoms like hives, trouble breathing, swollen face, unusual lethargy, itchy eyes? Another thought.. was there any dairy in the rice cereal? Somecof them have dried milk powder. That's how i figured out my son was allergic to dairy. Maybe dairy in the biscuit too? Without more info, it's hard to tell. Anyways, i'd be happy to brainstorm!