My son lost a tooth on his bottom row. It was the second tooth on the left from his front teeth.
He lost his second tooth from his molar on the top row.
「前歯から左に二本目の下の歯」は一文に入れるには情報量が多すぎるので、次のように言うのが良いかと思います。
ーMy son lost a tooth on his bottom row. It was the second tooth on the left from his front teeth.
「息子は下の前歯から左に2本目の歯が抜けた」
奥歯らか２番目と言うなら、次のように言えます。
ーHe lost his second tooth from his molar on the top row.
「彼は上の奥歯から２番目の歯が抜けた」
molar「奥歯」を一緒に覚えておくと良いですね。
ご参考まで！
カナダ 
