I'm not sure if this is the same on all routes, because I haven't played the Chapter 12 content on the beach, on any other route yet. But on the cuck route, Holly tells Ian that she's fallen for Lena, and she now feels the same way for her, that she used to feel for Ian. Which suggests that her feelings for Ian have changed quite a bit, since she started taking Ivy's guidance. It doesn't sound like she's in love with him anymore, and those feelings have now focused on Lena.
I also had him believe Jeremy's claims about Lena fooling around, and on this route at least he doesn't appear to be devastated by that possibility. He's pretty much guessed that it's Mike and Axel, who she's possibly fooling around with, and has decided to calmly consider those possibilities, before confronting Lena. Which shows how much he's changed, since learning that Gillian was cheating on him. He also very calmly accepts Holly's confession that she now has the hots for Lena, and that they kissed. Everything suggests he's not going to fall apart, like he did with Gillian, if his suspicions prove to be correct.
On this route, I have Lena pretty much succumbing to Seymour's machinations, so I highly suspect she's ultimately going to dump Ian for him. It's going to be really difficult for her to resist Seymour, if he offers her fame and fortune. And as he appears to be molding her to become his future partner and heir, this version of Lena, will almost certainly give in to such overwhelming temptation. Which means Ian may ultimately need a fallback position, even he goes along with Lena cucking him out.
The Cindy route has come and gone. So we can definitely rule her out. His relationship with Alison is really low, after repeatedly rejecting her advances, in favour of Lena. He never started a relationship with Jessica or Minerva, and he never flirted with Ivy, so she's just an aquaintance. Which leaves Holly or Emma. At this point, it doesn't look like Holly is going to want that kind of relationship with Ian, but that might change. Whereas, Emma is the only person he has maximum relationship with, and she secretly harbours an attraction to Ian, that she's never mentioned ( on this route at least).
Holly also could change very much in future updates, since she's on her slut route in this playthrough, so I'm very unsure how her relationship with Ian is likely to play out in the future. So even though, there's nothing romantic between the two of them at the moment ( on the cuck route), I have a sneaking feeling that something will eventually open up between Ian and Emma, later on in the game, particularly if Emma finds out about Lena cheating on him, and Lena's relationship with Seymour drives much more of a wedge between her and Lena.
The only question mark against that possibility is whether Perry can start something with Emma or plumps for Cherry instead, with whom he recently shared a kiss, even though he constantly downplays it. Maybe I'll have Ian persuading Perry to concentrate on Cherry, if he gets the chance. Whilst unconsciously flirting with Emma, everytime that becomes a possibility.