Who was the old friend Holden made at Elkton Hills?

Elkton Hills is one of many schools that Holden attends before winding up at Pencey Prep. Like all the schools he attends, it's a place that Holden cordially loathes, not least because—surprise, surprise—it's full of "phonies." According to Holden, the biggest phony of all at Elkton Hills was Headmaster Mr. Haas, whom he describes as ten times worse than Dr. Thurmer, head of Pencey Prep.
While at Elkton Hills, Holden didn't make any friends, but he did have a lot of time for a classmate of his called James Castle. James was a nice guy who Holden set apart from all the "phonies." Holden only talked to him once, and that was when he lent him his turtleneck sweater. James was wearing that sweater when he threw himself out of a window to his death to escape a gang of bullies.
Seeing James's bloody, broken corpse lying on the ground outside affected Holden deeply, dramatically changing his whole outlook on life. Ever since that terrible, traumatic day, when he saw what can happen to someone who dares to stand up to the "phonies" by whom he's constantly surrounded, Holden has often contemplated following James's example by committing suicide.