My Son Is 7years Old. He Has Hypoplasia Of The

My son is 7years old. He has hypoplasia of the corpus callosum. He has global delays. One of his issues is diarrhea. He will have 3-4 good days of regular bowel movements and then explosive diarrhea for 2 days. We have seen a gastroenternologist and there has been no medical reason. I believe he has IBS. What do you think. We have tried the bland foods, rice, carrots and apples. That works then we slowly introduce new foods into that. How many weeks should we give the newly introduced food to see a change in the bowel?