USAAF records indicate 147 B-29s lost due to flak or fighters. Broken down, 74 were lost to fighters, 54 to flak, and 19 to a combination of enemy actions (fighters and flak). 267 B-29's were lost due to accidents and mechanical failure (in-flight fires, fuel exhaustion, etc). About 10% of these are classified as being lost to unknown cause. This includes those deployed in the CBI theater.
Japanese pilots claimed 714 destroyed, 456 probably destroyed and 770 damaged.
I don't, for a second, believe that this guy shot down 9 B-29's.
If the ratio of over-claiming applies, he may have gotten 1. Probably a head-on ram, because the B-29 is faster than the Ki-61 at 32,000 feet, which can barely maintain altitude that height.