The way Ianto's been acting, Jack rather expected to feel hands on his head, pulling him in, and the lack of it has him pausing for a moment, tilting his head up with a soft, questioning sound, before he realises Ianto's just leaving him free to do what he wants to.
Well, that's the sort of challenge Jack can't help but rise to! He leans back a bit so he can use the tip of his tongue to tease a large proportion of the surface area of Ianto's cock and then, when he tires of that, one hand slides into the open fly of Ianto's trousers to curl around his hip while the other holds the base of his cock steady as he basically just swallows it down.
((OOC: I need an icon for Jack giving head. He does it so often and I have no appropriate icon!!!))