There's every possibility that it's only Kieren's exhaustion that's making him so willing to keep the charm, but Sirius isn't about to question it.

He relaxes instead, letting out a slow breath as he holds Kieren tighter, resting his cheek against Kieren's head. "All will be well," he agrees even though he's still not certain he truly believes it. Perhaps if he says if often enough and with decent conviction, it'll become truth.

And, until then, he'll just hold onto Kieren as tightly as Kieren will allow.