For cat the rat is the witness
is based on the idea that a cat is a higher, more trusted, animal than a rat.
If the father didn't believe his son in the first place, he didn't believe him any more just because his mother corroborated his story. If we don't trust the cat, we won't believe the rat.
We could say
If I don't believe him, why should I believe you?
but said to one's wife this could lead to domestic disturbances.
Another phrase, which indicates that the last statement has added nothing to our knowledge is
I'm no wiser than before
which in that context would indicate that he has no more confidence that his son did his reading now, after his wife had spoken, than he did before she had spoken.
This can also be phrased:
I'm as wise as ever.