If your son is not having a qualifying educational background as required for the post then in what way you think you can get any legal help on his.
Your son has done graduation in Pharma whereas the basic requirement for the post is graduation in Economics.
The PG qualification is a desirable qualification only and not a mandatory requirement.
Therefore the management is right in rejecting his candidature for not possessing the required qualification.
However you have a last line of hope if you would like to pursue the matter legally by filing a writ petition before high court seeking remedy and relief on the basis of passing the written examination conducted by the bank who should have rejected his candidature before allowing him to appear in the written examination.
Now, since the bank management has failed to scrutinise the educational quantification certificates before sending the call letter to appear in the written examination, you can hold them responsible and liable to provide an opportunity to him for the advertised post by permitting him to appear in the interview and have to present a strong arguments on the same lines by adding some citations in your favor in the writ petition before high court.