You can run by a petstore and buy some dove mix seeded bird food.
For the time being that would be fine. Just offer him other pieces of greens and breads, just see what he eats.

Pigeons are actually very smart creatures and can make awesome pets.
We have a pet pigeon that we rescued. His leg is forever deformed and we can never release him again, were ok with that though cuz hes our little friend who follows us around the house.