Ben was captured by Danielle Rousseau, who gave him over to Sayid (Ep: 2x14, One of Them). Kept as a prisoner in the hatch, Sayid, Locke, Jack, and Ana-Lucia tried to get answers out of him, but he would only say that his name was Henry Gale.
Sayid discovered that the real Henry Gale was buried in the jungle, but Ben still wouldn't tell them his real name. Later, Michael helped him escape in the hopes of getting his son back.
Hurley, Jack, Kate, and Sawyer were later captured by the Others, and there they saw that Ben was some kind of leader.