You can try feeding it wet bread (pigeons don't eat worms). Gently open its beak and push a small piece of wet bread down its throat. I use wet bread so that the bird gets a drink of water as well as food. The first couple of days I do this every few hours (not at night though!). I aim to get roughly a slice of bread down them.
If it survives you can feed it corn, you can get poultry corn from the pet shop.
It may have already died of shock, but if it has not then you could probably try releasing it in about a week (providing it has all its feathers and can fly!).
Pigeons often leave the nest a couple of days too early. They can usually overcome any injuries (never bothered with a vet myself - they will probably just wring its neck!).
Some seem to die even though they appear not to be injured.
The best you can do is try and feed it for a while and let nature take its course.