Gstr 3b of contractors

I have one party , he is contractor. Usually we filled ITR-4 (assuming 8% profit of receipts) of the same. He has taken GSTIN and now he is planning to file GSTR-Returns. My questions are as follows:

1.His total turnover during the year is less then 1.5crores . can he now opt for monthly return?

2.Monthly return is of what ? GSTR-3B? OR GSTR-1? OR Both?

3.What should i fill in expense sheet? Can i leave it blank ? to avoid RCM?