There's a heartstoppingly long moment there where he's not sure if he's gone too far, but the fierce grin on Spike's face reassures him and he grins back in relief - for a moment, at least; then his mouth falls open on a breathless cry as his cock's taken into a vicious grip, rough enough to make him buck again, rough enough to make him shudder with pain and arousal.
He laughs at the jibe, because 'pervert' is starting to feel like more of a term of affection than anything else, coming from Spike, and because he knows he's perverted by Spike's standards. It doesn't bother him. "Yeah!" he laughs breathlessly, and then he's jerking again with a gurgling cry, as Spike finally goes for his neck, properly goes for it, razor-sharp teeth slicing deep into his shoulder, and pain and adrenaline surge through him in equal measure. He clings onto Spike as he feels him start to drink, horrible and exquisite and amazing and agonising, determined to value every instant of this experience while he can.