Dear Kanika,
Pl.go thro' the attachment for definition of "Wages/salary" for ESi.
The ongoing ESI contribution should continue till the close of the half yr. period if his salary has been increased > Rs.15k in between. Supposing his salary was increased before 1.9.12 then there is no need to continue from Sept.12. On the other hand his salary increased > Rs.15K from 1.9.12 then U have to continue upto 31.3.2013 and discontinue thereafter. Is't clear.
For any clarification U can check back ESI web site or citeHR links where this subject was already discussed.
Regards,
kumar.s.