I felt so bad for him because he looked so much younger than his actual age.
この場合、次のような言い方ができますよ。
ーHe looked so young, it was kind of sad.
「彼はとても幼く見えちょっと哀れだった」
ーI felt so bad for him because he looked so much younger than his actual age.
「彼は実際の年齢よりかなり若く見えたので、すごく可哀想だった」
ご参考まで!