Who is the old man at the end of Nijigahara Holograph? At the end of Nijigahara Holograph (Rainbow Field Holograph), Amahiko meets an old man at the hospital who gives him the box he carried around for the whole manga containing "a spell". The old man then asks Amahiko what his name is and tells him he is called Amahiko too, just before turning into a butterfly (or so it seems from the last scene):



However in the whole story there isn't any other character named Amahiko so I'm confused about who the old man is, also because I've read various theories on different websites.

The old man and Amahiko could be the same person, however this would create a loop since  Amahiko (old version) would give the magic box to himself (young version) and wouldn't really explain why does he seem to turn into a butterfly.

Another interesting theory is that the old man is God (or some kind of supernatural being) who came on earth to encourage Amahiko who was kind of depressed and didn't want to stand up and recover from the fall, this is supported by the fact that he seems to know a lot about Amahiko (however this is also a good point for the first theory), his face is never shown clearly and he just disappears/turns into a butterfly (and the butterflies have a lot of supernatural meanings in this manga).

There's also the possibility that Amahiko, being lost in his own world, imagined the old man, but he is shown talking to another character at the beginning of the manga so I don't think this is the case.

So, my question is, who is this old man? And whoever he is, what does this character mean/represent? What does the magic box represent since it isn't actually opened by Amahiko?

I really like this manga, but there are so many details I need to clear up a bit!
Basically as ʞɹɐzǝɹ has mentioned in the comments, the old man is the older version of himself.

There's actually some really detailed discussion here.
Here are some quotes from that discussion that references Suzuki Amahiko as the old man.

Noting the old man turns into a butterfly but calls him Suzuki:


  Yeah, that's a great point about the same thing happening with Suzuki (and note that Suzuki Amahiko the old guy turns into a butterfly at the    end)!


A reply to monkey-boy detailing Suzuki meeting his past selves:


  
    Quote from monkey-boy
    
    
      p237: I take it that this is the butterfly saying goodbye, not K; which is to say that it is momma talking. It was also the butterfly talking, then, when K found Amahiko in the snow and said "but here you are, still alive" (p206); note that old-Amahiko also comes in the form of a butterfly (see p292) when he says essentially the same thing, "yet, in the end, you always wake up, and it's just you" (p008). 
    
  


A deeper analysis of its meaning was also written, here referencing Suzuki:


  He is an old man who speaks to himself as a child, a child who hands the box to himself as an adult, an adult who sees himself as an old man, and a butterfly who appears before his myriad human selves.


And a summary of events here, which lists the events in order, explicitly stating that he was the older version of Suzuki Amahiko.


  
  At the same time Suzuki jumps off the roof of his school, Arie is pushed down the well by her classmates.
  In the hospital he is given the tin box by an older version of himself. He is then transferred.
  On Suzuki's first day there aare two empty desks. The he goes to first would be Arie's desk.
  


If you still are unsure you can read the discussion there.