Yeah, I agree. Pre- and pro-biotics are for all of us and especially after something like you have experienced with your children. Also consider preparing some nutritious broths that will give him good nutrition that will go easy on the digestion as the enzymes and flora are restored.
Simmer some lean meat with lots of vegetables and some grains like barley, some onions and fresh garlic in lots of filtered water. Cover and simmer or slow cook until all is done. Then strain out maybe a cup of vegetables and six cups of broth. Puree the cup of veggies and grain and stir back into the broth. Don't season it; just give it to the boys to sip from a bowl or in a sipper type cup or insulated travel mug with a straw. Make sure it is comfortably warm. When they eat the broth or any meals you may prepare, make sure that they do not drink anything chilled or iced. This will "put out the fire" that the stomach needs to digest properly and will help prevent gas and indigestion. Let them have something warm or room temperature. The apple cider vinegar idea is a good one. I would put a teaspoon (based on their age) of apple cider vinegar (get Braggs or one that is unfiltered with the mother in it) with one or two teaspoons of honey and mix it with either water or some natural apple juice that is unsweetened. Warm it a bit and it will taste like hot apple cider. It is very good and will help aid digestion rather than hinder digestion.
A rice congee is good after diarrhea and vomiting. Put a sup of whole grain rice in a lot of filtered water, Bring to a boil and ten reduce and simmer all day in a slow cooker of stove top covered. Stain off the rice water and let them drink it. It’s very nutritious and the starch help settle the stomach and sooth the lining of the intestines.
Hope all this helps. I raised my babies in a third world country where diarrhea is the leading cause of death so I learned much of this there. The treatment is the same for adults.
Hope this helps.