I agree with the post above, and I'll add my two cents - In my opinion, IBS is the name for a collection of symptoms, and doctors tell you that you have it when they don't know what's wrong. Don't settle for this diagnosis because it doesn't really give you an answer on how to fix the problem.
It sounds like it could be either Celiac or a gluten intolerance. Check that a full Celiac panel was done, and if you can't get further testing try going 100% gluten free and see if it helps (but only after all testing is done). It could also be a different food intolerance or multiple (I know I'm sensitive to eggs and possible soy). Gluten seems to be the culprit much of the time and is a good place to start.
Remember that you are the best advocate for your family, so keep learning and asking questions. I've found that you cannot always depend on doctors to know what is best.
Good luck and keep us posted!