His mothers diet is less an issue than, the C-section and the antibiotic use. I would see if there are any probiotics for 18 month olds, things like non-dairy yogurt, and other live culture foods, add soaked chia seeds, flax seed, to his diet to enhance butyric acid production (prebiotics). Herbs to heal the gut mucosa include plantain leaf (tea), turmeric (in food), chamomile (tea) and they are mild enough for an 18 month old child.