Best milk for gaining weight

Hi mommies. I am a 9mmonths postpartum. 36wks lang si baby pagkalabas, considered as preterm. 2 kilos lang kanyang birth weight. Ebf aiya hanggang sa bumalik na ako from maternity leave. Advice ng pedia ay prenan milk dahil rich in protein daw, good for gaining weight. Mix feeding sya ngayon. Ngayon na 9mos nalang si baby ay 6.2 kilos pa din siya. Super baba ng weight compared to his age. I asked his pedia mag change ba ng milk since underweight, e sabi nya yun lang daw. Ako ang nababahala nga weight ni baby. Puree, lugaw, fruits and vegies lang kinakain nya araw-araw. Di nga nakatikim ng processed foods ever since. Question po mommy, any idea ba't slow pag gain weight nya? And we're planning to change his fm. Any idea din po ano maganda na fm for gaining weight? Thank you. #gainweight #formulaadvice