What is the ruling on a congregational prayer consisting of an imam, a man, and a child 12 years of age who makes mistakes in the pillars of prayer, and is it considered an individual prayer for the man?

If a person stands in a row with a boy behind the Imam, if the boy is discerning, he has the ruling of an adult and there is no problem. But if he is not discerning, then it is as if the one who stood with him prayed alone.