Urgent Help - Baby bird abandoned, chirping and wont eat, what do we do?

Hi there,

My partner and I had a baby bird fall out a high nest in front of us whilst walking home, near a main road. we didn't want to leave it as it was clearly injured. we have popped him in a box with air holes, cosy bedding, water and some food. He is constantly chirping and we aren't sure whats wrong. Any advice? All RSPB and local wildlife rescues are closed as its bank holiday. Advice really appreciated!!