Yes, Ianto has been enjoying himself but in the back of his mind has been that contant worry about Spike. If the atmosphere of this place didn't help keep him calm he would be a lot more frantic on day two than he already is.
Even so Jack's question catches him off guard and he looks surprised and guilty for a moment before controlling his expression. He hasn't mentioned seeing Spike yesterday, or the soul, or the fact that he told Rebecca and Nico but not Jack. The whole situation has him feeling nervous and uncertain, because he trusts Jack and he knows how far he and Spike have come but he also remembers Lisa, remembers what happened with Spike last time. If anyone will make the hard call about doing sometime to hurt him in order to protect everyone else it's Jack and even though he normally admires that trait about him today he's terrified about how it could all play out. He's sure Jack's noticed his nervousness, the pause, but he decides to try anyway, keeping things vague. He doubts he'll be able to get away with it. "I'll probably go see him today."