My 1month old down with flu

Hello mummies, my 1 month + baby got a block nose/runny nose and cough. We visited a normal doctor not a paediatric doctor and was prescribe medicine for him. I heard from my SIL that baby should not have medicine below 3month. So I’m kinda confuse. Can anyone advice or share your experience? 🙏🏼🙏🏼🙏🏼