Gst: show cause notice for cancellation of registration

Hello Good People

I hope you all are doing great.

My client has got a show cause notice, can you please help me how to reply to this notice.( Below is the Notice)

He used to file service Tax return till June 2017 as his Taxable Income is around 13 lacs (He is a Teacher in an Institute in Mumbai) but after migrating to GST he stoped raising invoice with GST because his Income was below 20 Lacs. 

Thank you.

............

Whereas on the basis of information which has come to my
notice, it appears that your registration is liable to cancelled for the
following reasons:-

1.      Non filling of Return for the following period 
JULY-2017 AUG-2017 SEP-2017 OCT-2017 NOV-2017 DEC-2017 
NF NF NF NF NF NF 
JAN-2018 FEB-2018 MAR-2018 APR-2018   
NF NF NF NF   
You are hereby directed to furnish a reply to this notice within seven
working days from the date of service of this notice.

You are directed to appear before the undersigned on 16/07/2018     at
11.00 AM. If you fail to furnish a reply within the stipulated date or
fail to appear for personal hearing on the appointed date and time, the
case will be decided ex parte on the basis of available records and on
merits.