GST Gst registration

One of my client's turnover is less than rs. 20 lakhs and 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 if he gets his GSTIN on or after 31 Aug then he has to file GSTR 3B and GSTR 1 for Jul and Aug'17 or not???