A 2-year-old boy was recovering Monday after surviving a 30-foot-fall from a third-floor window at a Revere apartment building. A Revere police spokeswoman, Sergeant Amy O’Hara, said the child ­remained hospitalized at Massachusetts General Hospital. The boy and his parents were visiting residents of an apartment on Revere Beach Boulevard when he apparently pushed through the screen on a sliding window and fell to the ground around 8:20 p.m., she said. An off-duty emergency medical technician who happened to be in the area provided first aid, O’Hara said. The incident ­remains under investigation by police, and the state Department of Children and Families has been notified, O’Hara said.