Runny tummy from antibiotics

Hi all
Ethan had a tonsilitis and on Monday the pead prescribed antibiotics which worked liked a charm. Only problem is that its giving him a runny tummy, despite the probiotics which I am also giving him. I know that one is supposed to just ride out a runny tummy, but really I am at a point where I have decided feck it, if I can take something for a runny tummy, why must my LO just ride it out.
Its pretty bad - we are back on nappies and this morning it actually just ran straight out of the nappy.
He is much better now, but he still has to finish his course of antibiotics, which will only be on Sat - so any suggestions on how to make the tummy stop??