Alas, given how these sorts of stories are structured, that won't be resolved until a big "he confronts her about his trauma" moment to get everything out in the open.
Something will happen to trigger his PTSD—maybe Izumi and the brother will run into each other at the mall, and then Tohno will spot them really hitting it off (probably by talking about him, but he won't know that); perhaps they'll be trying to plan a surprise birthday party for Tohno and Izumi will innocently ask for the brother's contact information so they can rope him into it; could be anything, really—and it'll explode for a couple chapters while he works through it all, has his emotional confrontation, and they resolve it.
