Grant uses his Army training to wriggle free of his wrist bonds before coming over to help Jason free himself.
At this point, Grant crawls back to his corner of the cage and tells Jason to attract the guard; he does so by yelling until the guard comes over.
When the guard pokes his head in between the bars, Grant grabs him and smacks his head three times against the bamboo bars of the cage.
The guard is either knocked out or, less likely but apparent to Jason, killed.
With that, Grant opens the cage and Far Cry 3 begins in earnest.