Revan would know better than anyone (except Tobias) about how to get rid of Nihilus. Besides, his son deserves to know so he's not caught off-guard.

"He can still use him, even if he can't sap the Force from him. It just means that Kale would have to suffer longer."

He does notice! He also notices the stance. "He'll try to force you. But I'm glad to hear that from you. Keep being stubborn like that." Axis gives his head a shake. "Tobias will probably try to kill him. And he'll probably want you to assist, Kale; you're one of the few people that would be able to."