My baby ate expired rice ceral, should i be concerned? he isnt showing any signs of discomfort. i noticed the date after when looking thru the box.

Unlikely: The expiration date on food products is more meaningful for taste and consistency (freshness?) than safety. As long as there was no bugs in it, it should be safe.

Don't make it habit: It most likely will not make him acutely ill.However,if you were to give him the whole box of it you would be negating the whole purpose of feeding your child nutritious food because expired food,including rice cereal,becomes decreased in it's nutrient content the older it gets,and therefore you aren't providing your baby with proper nutrition.He would be missing out on key vitamins and minerals