One who deals with interstate supply of goods and services under gst. He must get registered under gst. They cannot avail exemption. But yes they can choose composite scheme under gst for small businesses whose turn over is below 50 lakh.
Since he is purchasing from outside state he better register so that he can take input credit of IGST. if he does not register (as turnover is below 20 lakh) then still he can purchase but entire IGST will become his cost.