The Doctor says he knows, and Jack is certain that he does. After all, the Time Lord was in love with Rose when he met them, and he has two hearts invested. Both are probably breaking.
Jack isn't sure where he left his.
It takes a moment for him to realize that the Doctor has shifted closer and is holding out his hand. He stares at it, as if he isn't sure what to do.
Jack is still recovering and as he finally reaches out to take the hand, his forearm pulses and bones are set right, shifting under the skin. He hisses - it hurts - but he grips the Time Lord's hand and looks at him across Rose's body, uncertain what to say.