He's damn near tempted to vamp out, to really bite but for the time being he keeps it in check — barely. It's a good reaction regardless and Spike closes his fingers tightly around Jack's cock, giving it a few deliberately slow pumps to see what else he can squeeze out of him. It's hard work, keeping it steady while also gnashing teeth and lips and heat, and so he breaks the kiss after another beat, staying close enough to feel the warmth of Jack's breath against his face.
"So bloody eager."
Spike gives Jack's cock another firm squeeze while ducking his head to focus on his neck, blunt teeth finding skin to nip at even with all these clothes.