my 1 year old formula fed baby has very watery stools. it's been 2 days but it's only once a day. should i be concerned?

I would not be concern if it's only once a day. But do observe for more signs.

Does he have any other symptoms? Do ensure that baby is eating and drinking well.

Nothing to worry unless it happens more then once. what colour are the stools?

Please keep baby hydrated that's most important.