If his Aggregate Turnover is upto Rs. 50 lakhs, then your client can opt for Composition Scheme U/s 10 of the CGST Act 2017 and pay GST @ 6% ( 3% under CGST + 3% under SGST ) out of his own pocket.
Service Providers having aggregate turnover upto Rs. 50 lakhs in the preceding financial year are eligible to opt for Composition Scheme w.e.f 01/04/2019 as notified vide Notification No. 02/2019 - Central Tax ( Rate ), dated 07th March 2019.