Yea, it seems like he is schizophrenic.
It's not schizophrenia. Just because he hallucinates doesn't mean it is. If you look up Gestalt, you'll see that it is a theory in which the mind comes up with a perception/reality (gestalt) separate from other independent parts. It is a form of psychology developed by Kurt Koffka. In this case, the seme formed a Kei outside of the real Kei and according to the theory, the imagined Kei is greater than everything else, so it seems like that's why he was able to perceptualize him in such a way that he could interact sigh him. However, when he no longer believed in that Kei and started coming back to his senses, he broke his perception and thus, the conjured up Kei disappears.
Also, the original manga of this name is Gestalt (ゲシュタルト), which is why I told you to look it up if you're confused on what is going on in the story.
illusion like skizophreni?