His English was so good that people thought he was a native speaker.
His English was so fluent that people thought it was his first language.
ご質問ありがとうございます。
① "His English was so good that people thought he was a native speaker."＝「彼の英語はすごく上手かったから、人々は彼がネイティブ・スピーカーだと思った。」
② "His English was so fluent that people thought it was his first language."＝「彼の英語はとても流暢だったので、人々は彼にとって（英語は）第一言語だと思った。」
ご参考までに。
His English was so good, some people thought he was a native English speaker.
His English was comparable to a native English speaker.
His English was just as good as most native English speakers.
この場合、次のような言い方ができます。
ーHis English was so good, some people thought he was a native English speaker.
「彼の英語はとても上手で、ネイティブと思った人もいた」
a native English speaker で「英語を母国語とする人」
ーHis English was comparable to a native English speaker.
「彼の英語はネイティブに匹敵する」
to be comparable to ... で「…に匹敵する・…と同等の」
ーHis English was just as good as most native English speakers.
「彼の英語はほとんどのネイティブと同じくらい上手だ」
just as good as … で「…と同じくらい上手い」
ご参考まで！
He was mistaken for a native speaker because his English was so good.
英会話講師のKOGACHIです(^^)/
おっしゃられている内容は、言いたいニュアンスによっては
He was mistaken for a native speaker because his English was so good.
「彼の英語はすごく上手かったので、ネイティヴスピーカーに間違われた」
のように表現しても良いと思いました(*^_^*)
以上ですm(_)m
少しでも参考になれば幸いです(#^^#)
お困りの時は、いつでも気軽にご質問ください(^0^)/
★★どうぞよろしくお願いもうしあげます★★
see you soon♪
アメリカ合衆国 
カナダ 
日本 
