The reason why he is not firm it's because he is actually not his mother... he was a college student not too long ago. You are kinder with others' children that you are with yours: because it ain't yours.
Except Min already sees Toma as his own kid.
It does not matter, "he is actually not his mother". He is just random college student with a child who once forced motherhood on him. You don't become mother out of the blues. Adults are usually easier with others' children since they know "boundaries".
He doesn't need spanking. His mama just needs to be firm with him. Min gets easily swayed by Toma and that's not always good.