Evasion of GST

Good day all !

I have entered into a contract with builder for the construction of residential house in June 2017 at flat rate of 1500 per / ft

In July 2017 GST was introduced by Government. Although, the flat rate included taxes, He neither registered for GST nor paid any GST to Government or vendors to buy material for house and provided fake invoice in spite of doing business of more than 40 Lacs in 17 ~18 FY.

I want to lodge a complaint against the builder for tax evasion and want strict action taken against him. What is the best course of action ?