I very recently saved a sparrow which was about 3 days old. Birds such as sparrows need protein because they grow so quickly. Sampson (as I call him) is happily living in a friends garden. Pictures would give a clearer view on your sparrows situation/predicament. I fed him on special soaked formulated pellet (not available in stores unfortunately..) and defrosted pinkie maggots ( they aren't like the 'hollow' mealworms you get in shops, they have more substance and are a perfect size for sparrows.)
Crop milk is only usually produced by doves and pigeons, and flamingos, I think.
Sorry I'm not more help as the stuff I fed him we already had at my workplace and you have to order online if you want them. However, look up sparrow handrearing as there are a few really good articles out there, such as starling talk.com. There's also growth pictures so you can see how old your sparrow is.