LO Stomach Flu

Hello! Need help from the mommies here. My LO is 5months old and down with stomach flu, he has diarrhoea for more than a week (day 10 today) now. Seen PD twice but nothing they can do, given probiotics and diarrhoea medicine, also has changed his formula to soy based even tho he is mostly on expressed breast milk. His diarrhoea is only a small amount everytime he poop, but prior to this he always constipates so we are pretty sure is diarrhoea. Poop about 6-9 times a day. He poop so much until we are not sure is it his norm now or he is still having diarrhoea. Very worry about dehydration too, what should I look out for? How long does baby diarrhoea usually last? Thanks!