He often slurs his speech, so it's hard to hear what he's saying.
He doesn't enunciate his words, so it's hard to understand what he's saying.
この場合、次のような言い方ができますよ。
ーHe often slurs his speech, so it's hard to hear what he's saying.
「彼はよく不明瞭に発音するので、彼が何を言っているのか聞き取るのが難しい」
to slur で「不明瞭に発音する」
ーHe doesn't enunciate his words, so it's hard to understand what he's saying.
「彼は言葉をはっきり発音しないので彼の言っていることを理解するのが難しい」
to enunciate で「はっきり発音する」
ご参考まで！
"He tends to slur his words so it can be hard to understand him."
"He tends to slur his words so it can be hard to understand him."
"he tends to ~”「彼は〜傾向がある」
"to slur his words" 「不明瞭な発音する」
"so it can be hard to understand him" 「ため声が聞きづらい」
[slur](https://eikaiwa.dmm.com/uknow/questions/24900/)についてもっと詳しく知りたい方はクリックしてください。
カナダ 
オーストラリア 
