Hello,
Please feel free to ask him to issue you a proper GST Invoice. You may check his name on the website by merely putting the GST number on the website www.gst.gov.in.
If the name pops up after putting in the number then he is genuine, otherwise not. Also make sure to make the payment via cheque/ bank transfer if he is charging you GST. (preferable)
Trust this clarifies your query.
Feel free to call / get back in case of further clarifications.
Thanking You.
Regards,
Rohit R Sharma
BCOM, FCA, LLB, CERT. FAFP