I don't have information directly from Japanese records, but some stuff that might be useful:
Aakamatsu was a very senior pilot, 1932 pilot training graduate who survived WWII. There are short biographies of him in Hata/Izawa "Japanese Naval Aces and Fighter Units in World War II" and Henry Sakaida's "JNAF Aces" Osprey series book. He did fly the Raiden fighter with the 302nd AG in defense of Tokyo near the end of the war, but this particular incident is not discussed in either book.
I believe the response you got on USAAF forum came from this Japanese language site, anyway it says the same thing:
http://www10.ocn.ne.jp/~kuushuu/B29-42-63567.html
The root site is this list of US bombers downed over Japan, with detail pages for most, but its own sources aren't given.
http://www10.ocn.ne.jp/~kuushuu/mia.html
This series of (again Japanese language) pages extensively summarizes info in the Maru Magazine "Special: Pacific Air-Sea War" series. I don't know what citations those actual magazines give, but this site agrees well with the Japanese official history in some other cases, at least. It says Akamatsu was credited with 1 B-29 destroyed and 2 others hit June 10. However it notes the total JNAF strength v B-29's and P-51's in this action as 107 Zeroes, 24 Shiden, 11 Raiden and 6 Suisei ('Judy' divebomber, interceptor versions), which claimed 17 B-29's destroyed. It says US sources give only 1 a/c downed outright, so it might not be possible to attribute this downing to a particular claim with any certainty, even if the original records were at hand.
http://pico3298.ameblo.jp/pico3298/t...000325982.html
Joe