Jack came over to John, put his arms around him and pulled him in for a quick hug - quick because he didn't think John would accept anything longer.
"Take care of yourself," he said seriously, holding John away from him by the shoulders. "And come back here any time. We'll try and get the Master moved somewhere else as soon as we can, because I agree that him being here won't be doing the Doctor any good. It's just a matter of finding somewhere."