why is this child on forumula.
also if he is at your house why are you giving him so much formula, does he not eat.
why can't you give him water

also is he having loose or diarrhea. There is a difference.
also, how do you not know that its not milk its acually fruit. i have had kids who eat alot of fruit and end up with loose stools all day long.Or juice, he could be getting juice at home too.

also, yogurt does help the digestive system, and if he does have some tummy problems then yogurt may help

also, did you know that tylenol or advil also will give kids loose stools. I've been down this road too.

you have 2 choices, either term them or deal with it. Ya it stinks, but this is what I would do. I would start sending the poopy diapers home so they can see what is going on.

has he ever done a stool sample, because the child could have worms too.

you also have to remember to not mix your beliefs with theirs either. Also, maybe a food journal would be the best for you and your family this way you will have an idea what he has eaten or drank.