What is required to be an "FAA-qualified helicopter aerobatics pilot"?
I think this depends on what "FAA-qualified" means: the FAA defines aerobatic flight in 14 CFR 91.303 but it doesn't issue any pilot certificates or ratings for aerobatics. I did find several references to FAA Form 8710‑7, Statement of Acrobatic Competency (e.g. here), but the form itself isn't even listed on the FAA's form page.

There is an FAA document called Guidelines for Pilots Seeking All-Attitude Training which states that there is no special certification required:


  [...] no separate certificate, rating or endorsement is required to
  specifically qualify an individual as an all-attitude instructor


And:


  Anyone claiming to be an all-attitude instructor must at least hold a
  valid Commercial certificate if working for compensation or hire. The
  instructor must also be a certificated flight instructor (CFI) to
  provide FAA endorsements. Other than that, CFI credentials are not
  necessary for anyone to claim to be an all-attitude instructor.
  Therefore, caveat emptor - buyer beware - applies when seeking
  instruction.


Although their definition of "all-attitude" isn't exactly the same as "aerobatic", the document seems to make it clear that aerobatic training is not directly regulated. And looking through a variety of aerobatic websites including the IAC one, there is no mention of any required FAA certification. Endorsements for tailwheel, high-performance or complex aircraft are often required, but that's because of the aircraft itself, not the aerobatic activity.

But this profile gives some clearer information:


  Once the helicopter was ready, the FAA needed to certify it in order
  for Chuck to perform stunts with it. Once the aircraft was certified,
  the next step was to also get him certified, which ended up being a
  complicated process. The FAA was not sure how they could certify him,
  so they turned to Rich Lee, chief pilot for Boeing, who evaluated him.
  “He wanted to make sure that I did have all my screws in my head and
  that they were not loose”, said Chuck, laughing.


My guess is that the FAA first issued a new airworthiness certificate and/or waiver for the modified helicopter. That's very common and it would be same for a home-built aircraft, for example. Second - and this is a guess - they had the Boeing guy evaluate him for the Statement of Acrobatic Competency that I mentioned above. Although I can't find it on the FAA site, it obviously existed in the past and if you look at this example from the EAA Warbirds site, you can see that the evaluation process was (is?) 'outsourced' by the FAA anyway, presumably because they have no experts available internally:


  I desire to take advantage of an FAA procedure for the evaluation of
  my competency by a Warbird Aerobatic Evaluator (Warbird Evaluator) on
  the list maintained by the EAA Warbirds of America, Incorporated.


I suspect that putting a unique airworthiness certificate together with a unique Statement of Acrobatic Competency resulted in a uniquely "FAA-qualified" acrobatic helicopter pilot. 

Finally, according to Aaron's LinkedIn page he is the:


  First FAA Certified Helicopter Pilot to recieve a Helicopter Aerobatic
  Certificate from the FAA.


That again seems to emphasize the helicopter, not the pilot.