Percy Jackson and the Lightning Thief brought in $88 million domestically in 2010. It made better overseas making Fox enthusiastic enough to make a sequel. It seems they were able to attract Logan Lerman with this enthusiasm.
Times reports that the studio has “exercised its option” on the actor, meaning they have the power to force him back for a sequel, and they are doing it.
On board is also a new director, Thor Freudenthal, that made a success with Diary of a Wimpy Kid. Lerman is already 19 and he probably wants some adults roles. So the studio is moving quickly and the next Percy Jackson could be in theaters as early as 2012.