Who is the old man at the end of Nijigahara Holograph?
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.