That's what the entirety of ch 5 and 6 were about, him telling Akane to stop pretending to be Mei since he's fallen in love with Akane now. He forces Akane to visit Mei's grave with him to give them both the closure they need to move on as themselves in a relationship rather than as Mei and him.
It didn't seem that convincing to me, though; Keisuke came to the realisation that he had, apparently, come to love Akane a little too suddenly but, at the same time, also way too late, in my opinion - it almost seemed forced to me - but to each their own! :)))
I see what you mean, it didn't SHOW us him falling in love. Personally, i was fine with all the hesitation and serious thinking he was showing while looking at Akane as he pretended to be Mei, it was like he really did decide to move on. That's just me though.
Their relationship seems pretty unhealthy to me; Keisuke should love Akane just the way he is rather than continually projecting his love for Mei into Akane. They're twins but they were obviously two very different individuals... ╥﹏╥ It would have been better if Akane and Keisuke had gone their separate ways after Mei's death, allowing themselves to heal emotionally.