Hi,
A couple of years ago whilst driving to our loft I spotted a wood pigeon that had been hit by a car, As I was driving upto it, I realised it was still alive, pulled over and picked it up, it had got a small cut on its wing and also a lump on his head, I put it in the car and took it to the loft, where I put in an aviary that we weren't using at the time, I gave it some water and pigeon corn in a pot and left it, I thought it would be dead the next day.
when we arrived at the loft the following morning it was still alive but hadn't drunk, so again we left it but put the corn and water nearer to where it was standing, over the next couple of days it slowly started easting and drinking, then eventually perching not once trying to escape, this went one for 2 or 3 weeks, and you could tell he was getting better as he had an exceelent appetite - one day when dad opened the aviary door to feed him, he flew over his head and to freedon. - our good deed for the season.
So, to cut a long story short, corn and water will be fine, try not to have in in a box, try to make the environment as open as you can, with the warm weather we are having lately I would be tempted to put some water with electrolights down his throat if he hasn't drank by the morning that is....
Good luck and let us know how you get on.......