That right there is exactly the problem. Sharing pain is just not something Inigo does with friends. Not because he doesn't trust them - he trusts Prompto for sure - but because he never wants to unload his pain onto others. He just can't. Inigo spent years and years surrounded by people who were suffering just as much as him, if not worse, and so he had to be the one to be able to suck it up and keep things together for everyone's sake. He had to learn how to smile in every situation. Which is exactly why he's here in a corner of the train to begin with. He was hoping to unload this all somewhere no one would find him, so he could return and smile with everyone again, no matter how much his heart ached on the inside.

Clearly that plan has failed though, and trying to hide it is a ship that has long sailed now Prompto is on to him. Inigo sucks in a deep breath. Maybe he can still salvage this if Prompto is clearly having a problem as well. He furiously rubs at his face to wipe away the tears, trying so hard to mentally will his crying to stop. Just don't think about Yuki. Don't think about how the endless stream of rejection just keeps going. Focus on Prompto for now. He can do it. Prompto's the one who deserves and needs some comfort and support right now.

".. I'm so sorry." His voice is weaker than usually still, though less choked now. "Of course that's got to be really rough for you, even if you'll see him again back home. At least you've still got the other guys, right?"