Taxability

One of our client did not pay service tax till jan 16 as usually his gross receipts does not exceed 10 lakh. though he had taken ST registration earlier. now in current year...his receipts are going around 12 lakh. my query is how service tax to be levied ? do he need to pay service tax on entire rs 12 lakh in this month of march 16 ? and do he need to file both half yearly ST returns ? kindly advise