Use an infant/child formulation, that is powdered.
Follow directions for his age, and weight, and mix
the BIFIDUS with
water. This could quickly normalize his bowel movements, if the problem was partly due to an imbalance of the large intestinal "flora"(bacteria)-- due to antibiotics, or other meds.
Also, avoid everything ARTIFICIAL. Read labels. And, check your water supply for contaminants, both micro-organisms, AND toxic chemicals!