Thank you for your query.
Do not panic .Theses are symptoms of reflux.
Feed your baby in a upright position.Give thickened feeds.
Give small amounts of feed at a time.
Burp the child well.Use Domperidone
drops before feeds for some days.
The child is underweight
.Consult your doctor about that.
If you have any further queries you are free to consult me.
Hope this answers your query.
Wishing your child a speedy recovery.
Have a nice day.