He looks almost stunned, like he can't believe this is Jack's decision. The expression immediately transforms into embaressment as he realizes how insulting that must be, the way it seems like he didn't trust him.
"Thank you. I'll tell you if the situation changes, or if it seems like he might be losing control." It doesn't seem likely. Getting that soul back on his own would settle a log of Spike's demons, both literal and metaphorical.