He took a few steps back the moment he realized he was about to be attacked by a bear.
He saw a bear and thought it was going to attack him, so he started walking backwards.
ーHe took a few steps back the moment he realized he was about to be attacked by a bear.
「熊に襲われそうと気がついた瞬間、彼は数歩後ずさった」
step back を使うなら、to take a few steps back「数歩後ろにさがる・後ずさる」のように言うと良いでしょう。
about to で「〜しそうである」
ーHe saw a bear and thought it was going to attack him, so he started walking backwards.
「彼は熊を目撃し、彼のことを襲ってくると思ったので、後ずさり始めた」
to attack で「襲う」
to start walking backwards で「後ずさりし始める」
ご参考まで！
カナダ 
