HI karthick,
there will be a standard event EH_ONSORT in standard IMPL class. When you click sort system will trigger only standard event until and unless you redefined it in your custom IMPL class.
You need to redefine the SORT event and use the similar code as in standard sort event method in addition to add your custom logic for sort based on your custom field.
Best Regards,
Dharmakasi.