Jack's not wrong about that — it is more and more a term of endearment, something he can say that doesn't betray that he genuinely cares. He does it with Ianto too, so it's not just to keep Jack at arm's length. Maybe it's something to look into later. Right now he has work to do.
Growling into the bite, Spike drinks and sucks and moves his fingers in tandem over Jack's cock, easing up on the roughness just a little bit. That time in the park, a life time ago, he could almost feel how close to bursting Jack had been and now that they have the chance of recreating it it'd be a shame if he didn't get a side order of a little death.