Gst on maintenance work done on completed works contract

A new client of mine,a works contractor,executed  and completed government contract for construction of roads before 1st July 2017. The five year maintenance work is going on and during the financial year 2018 - 19, he received Rs. 500000/- as contract receipt for maintenance work (3rd year maintenance) done during that year. Is GST @ 12% payable on this amount received? TDS @ 2% is deducted by the department. 

He also claims that 4% VAT has been already deducted on the original Contract amount.