New registration under gst

one of my client's turnover is less than ? 20 lakhs but he wants to supply to some government departments but they used to buy from registered dealers under GST, that is why he wants to get GSTIN but my question is that he has to file GSTR-3B for Jul and Aug'17 even after he was not attracted to tax under GST because his registration is voluntary basis, if yes, please explain it.