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.