I'm not sure what kind of handfeeding food you've got, but just to be sure: handfeeding food for parrots isn't suitable because their diet is very different. This page has a recipe for handfeeding food for sparrows in case it's needed:
http://www.starlingtalk.com/babycare.htm#dietThis link has additional information on feeding rescued sparrows:
http://www.starlingtalk.com/house_sparrows.htm It also has a link to pictures of baby sparrows at different ages so you can try to determine how old yours is. Millet spray is recommended for babies that are two to three weeks old, and millet is a major component of parakeet food, so it's completely appropriate.
It's very likely that mama sparrow is hanging around outside looking for her baby. Fledglings that are all by themselves usually haven't been abandoned and have a parent coming around regularly to feed them. So if the weather is better now you might want to put the baby back outside in the area where you found it, and keep an eye on it to see what happens. Here's more info on dealing with fledglings that you find:
http://www.birds.cornell.edu/allaboutbi ... s/orphaned