And here I thought some player fell victim to the infamous "Polymorph to Dairy Product" Spell
First,
Does EVERY character he makes act that way? If so, he's basically playing the same thing over & over. (Not that is bad, some people tend to enjoy playing one character race, class, alignment, over any other). Or, does each characcter have its own manner of "nuetrality" as the player wrote down.
For instance, does one character kill children, but would never hurt an old person; while another kills old people yet gives children candy? Look for methods to his madness from one character to another. If you see different patterns emerging, he may be playing more sophisticated characters than you realize.
As for his Ture Nuetral Alignment. Not by the PHB. He's acting evil. Alignment Options for what you've described.
Chaotic Nuetral (DM option): Some DM's rule the insane (or certain types of insanity) as CN. Characters commit evil acts, but are not trully evil, since they are incapable of realizing their actions. (The philosophy of insanity is widely debated today and views vary widely from one individual to another, so this alignment option can open a can of wyrms if all players doen't agree about the criminally insane being CN).
NE: IF he isn't going out of his way to kill children of butn down random buildings, he may be NE instead of CE. If he kills the children cause their "in his way" or burning down the building because they wouldn't serve him, or its run by an elf, or any of dozens of reasons. He may be evil, but he may lack the chaotic tendencies of true CE.
LE: A stretch here. Most of what you describe has little Lawfulness associated with it. Unless he's performing his acts of murder and destruction to some religious or personal code (if he is, make sure you have it write it down & give it to you).
CE: As most people here state. CE is most likely what he's playing. As you described, lacking further details, I'd call him CE. If the player has a problem with that, read the CE alignment description in teh PHB & list off how is character mathces the CE stereotype to a tee.
As an option, you Could have his character as CN with evil tendencies. But it sounds like he slipped over to full CE a long time ago.