But it wasn't a break-up, she disappeared, went missing, so his frame of mind was not "I'm sad cause I broke with a girl", but "I'm in mental anguish over not knowing what happened to the girl I love; is she dead, is she alive, is she tortured, kidnapped, joined a sect, raped, buried alive?". This kind of uncertainties and mysteries - especially in regards to a person that went missing at the peak of their relantionship - can really wreak havoc in human mind, even in the case of adults (I recommend 1988 film "The Vanishing" btw). I'm not saying that MC is mentally healthy or that the game is doing miracles with portraying human psychology, but the shock of such situation can be devastating for a very young person, whose personality is still being shaped, so I don't find his behavior all that unbelievable.
But that's just my take on the situation.