weight of baby is good as per age and birth weight, the symptoms described seem of viral origin. Should be improving by now if only viral infection. Give plain water after feeds to maintain good hydration. Hope the baby is breast fed.
Hope there is no fever.
Poor feeding could be because of nose block and simple aches and pains. Nose block could be relieved by saline nose drops. For aches and pains paracetamol drops are enough.
Next Steps
if not on the way to relief, should consult a paediatrician
Health Tips
As above