Most commonly, we would say:
Notice a 「んだ」 is used in the first sentence.
The vast majority of native speakers would not use a pronoun like 「彼は」. They might use 「あの人は」, but it is not needed as both parties know exactly who they are talking about.
Since Japanese is a highly contextual language as I say here every week, the exact phrasing of the answer depends heavily on how the question "Why did you call him Japanese?" was asked in Japanese.
If you used 「なぜ」, the other person is likely to reply with 「なぜって」. Similarly, a 「なんで」 question is likely to be replied with 「なんでって」.