He was captured with his father and brother by Túrin's outlaw band. During the Sack of Amon Rûdh, he was missing in action. He might have been killed by an orc or might of died of an accident like his brother. But after the destruction of Amon Rudh, he was never seen again.
Behind the scenes
In The Children of Hurin and Unfinished Tales, Ibûn is last mentioned being held hostage by the Orcs that Mîm had aligned himself with, but he is never mentioned again after that.
|Mîm | Ibûn | Khîm|