After getting Castel's health down a certain amount, he pulls out a knife. This gives Castel an edge in slicing Jason Bourne's pretty face. You can disarm him though, by moving the fight over to the desk. Once you are in front of the desk, perform a takedown. If Jason is in the right spot, he will stab Castel with a ball-point pen. Without the knife, Castel does much less damage. Note that you do need to do a takedown near the desk; any other location will allow Castel to keep his weapon.