just make ready his birth certificate attested by qatar embassy india and passport of u r baby, if his age is less than 90 days then u have to provide these two documents while entering to qatar. immigration at airport will enter a number on your babay birth certificate after that u can proceed for his RP.
but if u r baby is more than 90 days then you have to process same steps what u already done for u r family visa.

another advice to u , if u r are planning to take u r newly born baby by qatar airways then go to their office and get a form , otherwise they are not providing boarding to u r baby as the crew of qatar air ways are not aware about the entry of newly born baby procedure. so better first contact them in their head office.