Baby Blackbird

Hi all! 

I have a family of about 21 blackbirds living around my garden and they all feed on my feeders along with lots of other birds. 

This morning a cat got to one of the fledglings, I managed to get to the baby but he was quite weak, I currently have him in a dark big box with air holes and a warm towel. 

What should I do next? Any help appreciated! I want to reunite him with his mum ideally!