He's really getting good at English quickly, isn't he?
He's really making big improvements in his English and he's a fast learner as well.
この場合、次のような言い方ができます。
ーHe's really getting good at English quickly, isn't he?
「彼はすごく早く英語が上手になってきてるよね」
to get good at English quickly で「英語が早く上手になる」
ーHe's really making big improvements in his English and he's a fast learner as well.
「彼はすごく英語が上達していて、飲み込みも早いです」
to make big improvements in one's English で「英語がすごく上達する」
a fast learner で「飲み込みが早い人・覚えるのが早い人」
ご参考まで！
His English has really improved very fast, hasn't it?
It's crazy fast how good his English has gotten.
ご質問ありがとうございます。
この場合、「彼の英語はすごく早く上達してるよね」と言いたいなら、英語で「His English has really improved very fast, hasn't it?」と言えます。
また、もっとカジュアルな言い方を使いたいなら、「It's crazy fast how good his English has gotten.」という風に言えます。
ご参考になれば幸いです。
"彼の英語はすごく早く上達してるよね"
- His English is improving incredibly quickly.
- His English is improving at a really fast rate.
- His English is improving so quickly.
- He's getting better at English at a rapid rate, isn't he?
カナダ 
アメリカ合衆国 
