He's always absent-minded and answers you as though he's unconcerned.
He's not always there when you talk to him. When you ask him a question, he answers as though it is not his affair at all.
この場合、次のような言い方ができます。
ーHe's always absent-minded and answers you as though he's unconcerned.
「彼はいつも上の空で、他人事のように答える」
absent-minded「上の空の・ぼんやりした」
unconcerned「関係がない・無関心な」
ーHe's not always there when you talk to him. When you ask him a question, he answers as though it is not his affair at all.
「彼は話しかけてもいつも上の空で、質問をすると他人事のように答える」
not always/all there「上の空で]
one's affair「人のこと」
ご参考まで！
His responses are always absentminded and he acts as if it's unrelated to him.
ご質問ありがとうございます。
・「His responses are always absentminded and he acts as if it's unrelated to him.」
（意味) 彼はいつも上の空で他人事のような返事する
<例文>His responses are always absentminded and he acts as if it's unrelated to him. I'm not sure if he's actually listening.
<訳>彼はいつも上の空で他人事のような返事する。 本当に話を聞いているかもわからない。
参考になれば幸いです。
カナダ 
アメリカ合衆国 
