The A5M was a 1934 design and first flew in Feb 35 nearly a year before the He112 so it is unlikely there was any input into that by David as it was already probably an established design when he got there. If he did have input then it is unlikely the He112 provided any inspiration.
I was unable to find when he actually arrived in Japan but given how much money was being pumped into the German aircraft industry at that time ,and how fast a paper concept became a flying aircraft in Germany then I would expect that the He-112 was barely, or little more than, a concept when the A5M first flew.
The He 118 first flew 1936 and was prone to converting itself to components in flight. The Aichi D3A first flew two years later and did not have that habit. He may well have had input into that design.
I suspect that he may have been recruited by the Wackett team while they were in Japan in 1938 looking for aircraft for CAC to build and assessing Japanese designs and negotiating to make Japanese alloys under licence in Australia.