The boy was standing alone leaning against the wall.
A little boy was leaning back against the wall all by himself.
leaning his back against the wall という言い方はしないので、次のように言うと良いでしょう。
ーThe boy was standing alone leaning against the wall.
または leaning back against the wall とも言えるので、次のようにも言えます。
ーA little boy was leaning back against the wall all by himself.
「少年は一人で壁にもたれていた」
ご参考まで!