ducklings!

Help!!
We had our first hatch of ducklings last week, two are doing fine but one (the last one to hatch) is really worrying me.
He's a magpie duck, three days old. He eats and drinks with no probs but his balance is awful. He spends a lot of time on his back trying to right himself, and when he's up he just wobbles toward wherever he wants to go. He's had a swim and although he's lower in the water than the other two - he managed fine.
Has he got something awful? Poor little duck - he's trying very hard but what an effort to just waddle in a straight line!
Help! Any thoughts? Advice?
Thanks very much!

thanks! will do first thing tomorrow! fingers crossed for the little fella (or girlie) whichever!

Most chicke crumb is medicated, ducklings must have UNMEDICATED feed