"to absolve the simoniac of his sin. "

Cabe reports: Simony is committed by acquiring spiritual things in return for a remuneration [v, ref.]. In Dante's Divine Comedy, a simoniac, Pope Nicholas III is marooned in hell hanging upside down in a flame. Simony is such a great sin that Saint Thomas Aquinas in his 12th century Summa Theologica, tells us that "The Pope alone can grant a dispensation to one who has knowingly received a benefice (simoniacally)" for it is one of the most grevious sins. Father Flynn knew this basic rule, for, as he tells the boy, his concern with the tenets of the church is encyclopedic. (more)

So even if in some way that we haven't yet realized, Father Flynn is a somoniac, it still would be unrealistic to think that the Priest would confess to the boy, or ask for absolving forgiveness from him.

Yet this is a dream, and as Alice has reminded us, clearly this is the boy's dream, not Fr. Flynn's. But perhaps the boy is dreaming/intuiting some truth not only about his relationship with Father Flynn--as Alice is suggesting--but also about Father Flynn himself.  What are dreams, anyway.  Can't they give us insights about the actual world out there?