Can you post a pic of the bird? He may be injured or sick. If he doesn't start eating, you may need to hand feed him. You can do this fairly easily with frozen peas which have been defrosted and warm under warm water. How many peas would depend on his age or size. If he should suddenly be able to fly then he will get out of the box. Can you cover it with a grill or something that won't make it dark in the box?
If you need to feed peas to a pigeon, hold the bird on your lap and against your body. This gives you more control. Reach from behind his head with one hand and grasp his beak on either side. Now use your free hand to open the beak, and put a pea in, then push it to the back of his throat and over his tongue. Let him close his beak and swallow. Then do another. It gets easier with practice, and the bird also gets more used to it, and won't fight as much. If you can't handle the bird, then use the sleeve cut off a t-shirt, slip it over his head and onto his body, with his head sticking out. This will stop him from being able to fight you so much. Just don't make it tight around his crop area. It helps if you have him facing your right side if you are right handed.
Thank you for helping the poor little thing BTW.