Frankly, Jack's so used to getting a good response from this sort of thing, he'd be quite upset not to get any at all. The hardening of Spike's cock is gratifying, as is the shift closer, and he grins as Spike (inevitably) offers him a challenge.
"Yeah?" he counters, his eyes just as tight on Spike's as Spike's are on his. "What more would you like, Spike?" He shrugs slightly. "I'm not saying there's not more - and there's a whole load of stuff I'd love you to do to me - but so far all of this has come from me. I haven't heard much from your side of the equation yet."
Challenging him right back. It's not that he hasn't got more to give; it's more that he's still a little bit wary of crossing the line and scaring him off.