That he's healing, that they're still stuck in this dream is all the evidence he needs to know that Jack is fine. Spike disengages from the wound, lets the feral vampiric features melt back as he waits, holds on, licks his lips clean of the blood that's still dripping warm and wet down his chin. That was something else, he's aching hard and can't even care too much about it. Sometimes not getting satisfaction is the most satisfying.
The gasp surprises him but he gathers his wits about him quick enough, giving Jack a pat on the shoulder as he comes to.