Yes, the seed will sift through. He will find it. The box should only be a couple of inches deep. You should find all the info you need in the Dove and Pigeon Forum. Also, for parasites, a bath in warm water with a bit of Borax in it will do the trick. (rinse well) Sounds like I'm giving you a recipe.
Several of us have had the pleasure of giving Pigeon a home. The Dove is very much the same.
You will find much peace having that little one around.