He's a really popular teacher, so it's hard to get a lesson with him.
He has become quite a popular teacher, so it's tricky booking a lesson with him these days.
この場合、次のように言うと自然な感じで良いですよ。
ーHe's a really popular teacher, so it's hard to get a lesson with him.
「彼はとても人気の先生なので、彼のレッスンを受けるのが難しい」
もう少し日本語に近いように言うなら、次のようにも言えます。
ーHe has become quite a popular teacher, so it's tricky booking a lesson with him these days.
「彼はとても人気の先生になったので、最近は彼のレッスンを予約するのが難しい」
ご参考まで！
He's become popular and it's difficult to reserve his lessons.
Due to his increasing popularity, reserving his lessons is becoming difficult.
ご質問ありがとうございます。
① "He's become popular"＝「彼は人気になり、」
"and it's difficult to reserve his lessons."＝「そして彼のレッスンを予約するのは難しいです。」
② "Due to his increasing popularity,"＝「彼の人気度が上がっているため、」
"reserving his lessons is becoming difficult."=「彼のレッスンを予約するのが難しくなってきている。」
ご参考までに。
He has become so popular that it is now difficult to book his lessons.
おっしゃられている内容は、以下のようにも表現できると思いました(*^_^*)
He has become so popular that it is now difficult to book his lessons.
「彼はすごく人気になったので、今は彼のレッスンを予約するのが難しい」
以上です。
少しでも参考になれば幸いです(*^_^*)
カナダ 
アメリカ合衆国 
日本 
