Injured blackbird can’t fly

Hi,
I spotted an adult blackbird sitting on the floor across the road from my home late last night, it allowed me to pick it up and sat on my finger allowing me to touch him without panicking so I assumed he was in shock, he has some missing feathers at the back of his head so I think he’s been clipped by a car. I have put him in a well ventilated box with some fat balls and water, he was happy for me to take him out of the box this morning and I held him out to see if he could fly off, he attempted to fly and did manage to for a short while but couldn’t make it very far, can you give me any advice?