Underweight po kasi si baby nyo, so best to consult your pedia po, if you haven't already.
As for eating solids, offer solid foods po muna bago bigyan ng milk or other snacks, and don't give them milk/ snacks too close to meal time. Be patient and keep on offering lang po ng iba't-ibang foods, at iwasan muna ang mga pagkaing may salt and/ or sugar para hindi maging picky eater.
Tere SC