"He must be Japanese", if translated to idiomatic Japanese according to the standard concept, should be:
彼は日本人に決まっている。
What about the following alternatives?
彼はぜひ日本人です。
彼はきっと日本人です。
彼は必ず日本人です。
Are these acceptable as well?
彼はぜひ日本人です。 is not acceptable, I'm afraid. You use ぜひ this way:
ぜひ参加してください。 'please' (for invitation/request)
ぜひ参加させていただきます。 'by all means' (when accepting offer)
彼はきっと日本人です。 sounds fine. 'I believe he's Japanese' / 'I'm sure he's Japanese'.
彼は必ず日本人です。 is not acceptable. 必ず is used this way:
必ず来ます。 'without fail' (for future action)
寝る前に必ず歯を磨く。 'never fail to do, always' (routine)
人は必ず死ぬ。 'doomed to'
You can also say 彼は絶対日本人です。
I think 決まっている sounds rather casual and きっと conveys less certainty than 絶対 and 決まっている.
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5I think 彼は必ず日本人です。 is not perfect, but it is still acceptable. The sentence alone is a little strange, but in some context it sounds all right. Say, このパスポートを持っていたということは、彼は必ず日本人です。 So if someone has enough Japanese ability, he/she can understand the sentence. – Takashi May 9 '15 at 8:11
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Is that sentence natural enough to be taught beginners or intermediate learners to say? – user5185 May 14 '15 at 8:08
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Learning through such borderline cases is helpful, I think (to me at least). – user9771 May 15 '15 at 14:15
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1@user51185 If you say 彼は必ず日本人です。 it sounds like you have an objective evidence, where 彼は絶対日本人です。 doesn't limit to only an objective evidence, but it also sounds like subjective evidence. If you use 必ず unwiring context, it sounds a little strange. If I was asked if 必ず is acceptable I say yes, but if I was asked if 必ず is natural enough for beginners, I'd recommend them to use 絶対, because it can be used more situation. – Takashi May 16 '15 at 6:21
必ず必ずand連絡する連絡する(from your other question)? – Earthliŋ♦ May 8 '15 at 14:39