Kieren has made the mistake of closing his eyes, which means that he doesn't see where Sirius is putting his hands. It makes him cry out, a sudden thing that would have him coming if he had the ability. Part of him is now just grateful he can't get hard, because he'd be aching with it, pleading Sirius for some relief. As it stands, he's the one to offer it, so he swallows him deep again, eager to give them both a climax.