I'm going to put my child one grade below the grade he is supposed to be in.
He is going to study with kids who are one year younger than him because he doesn't understand much English.
「１つ下の学年に入れる」という言い方は下のように言えると思います。
1) I'm going to put my child one grade below the grade he is supposed to be in.
「息子を本来の学年より１つ下の学年に入れます。」
one grade below で「１学年下」
少し言い方を変えて、「１つ下の子たちと学ぶ」とも言えると思います。
2) He is going to study with kids who are one year younger than him because he doesn't understand much English.
「息子はほとんど英語がわからないので、１年下の子たちと学びます。」
ご参考になれば幸いです！
日本 
