Begin leaving the cage outside during the day so other sparrows can visit. If you are avoiding close interaction with the bird and it is interacting with wild sparrows, then its chances of re-acclimating to the wild are much better. If the baby bird is not interacting with wild birds then it needs to learn the songs of its species another way. This will allow it to communicate with other birds when it is released. There are audio files available online that you can play for the bird. Let it hop around on the grass after about 7-10 days old. If releasing the bird is your ultimate goal, try putting it in open space areas so they can learn to fly. Instinct will teach the bird to fly and discover what its wings are for.  Wait until it has wing feathers, then if it doesn't seem to know what to do then its probably isn't ready. To test out whether he's ready, take him outside and place him on the ground in an area that's safe from predators. Leave the bird to its own devices for 20 minutes, if nothing happens bring it back indoors and try again another day. If you're going to release it, make sure the baby can eat on its own. You also need to be sure that it is not imprinted on you. If the bird is imprinted on you, then it cannot be released into the wild. It needs to continue as a pet.

Summary:
Put the baby bird in a cage when it begins hopping. Give the bird more and more time outdoors. Make sure the bird is ready for release.