Hello pls check with your pediatrician immediately, my baby didn't have flu per se but slight nasal congestion the PD gave saline nasal spray to relieve the congestion. PD will also check if there is congestion in the lungs. Since baby is just 3 weeks better to see doctor and seek advise.
You can bring to NUH/KKH for NB they will check for you accordingly. However, please be inform for their age no medication will be given as to my knowledge