GST in construction

Assessee is a civil construction contractor. Assessee has not taken registration under GST in the financial year 2017-18. He obtained GST in the month of May 2018. But Gross receipts through Bank are Rs 89000000. How can he pay GST for the liability on Rs 89,00000 for the financial year 2017-18 and how can he show the GST liability in Balance Sheet.