GST Registration

he is not required to pay serivice tax because service tax is now ended. 

 The GST law itself grants the basic exemption of Rs20 lakh to the small taxable person. Every supplier shall not be liable to GST if his aggregate turnover in a financial year does not exceed Rs20 lakh.

So the registration under service tax is automatically covnerted to GST and need not separate cancellation. Continue your registration under GST law since your client do not have turnover more than 20 lacs he is not liable to pay any tax under gst

Effect on return:

once your client is registered voluntarily he must file returns every month even if his sales are zero. 

Exemption:

sorry i need to make correction in my above comment, The basic exemption of 20 lacs is not available aftre registration.  Once you register for GST either voluntarily or compulsorily you have to pay tax on 1 rupee sale and return must be filed. your client has to pay tax on whatever supply made. There is no exemption clause in return. 

but he can claim input credit if he sells further to registered deler. and wholesaler will sale goods to your client because wholesaler will get input credit by selling goods to your client. 

Concultion: take registration if your client purchases services from a registered dealer and that dealer is asking for GSTIN for his input credit. you can register as input service distributor.

If your client dont want to file rerunts every month then its better to cancel the registration and pay normal income tax.