Baby pigeon abandoned on my balcony

Hi everyone 

A pigeon built a nest on my balcony and hatched a chick 17 days ago. 

The mum has now left and the chick is still there.

The balcony has a metre high wallthat the chick would have to scale before trying to fly and the flat is on the 5th floor.

At the moment is just sitting in the corner.

What should I do?