How To Improve Appetite And Weight Of A Baby?

Hi, My baby is three and a half months old. From past few days he is not showing interest towards feeding. Whenever he is hungry he cries but after few sip he starts pushing the bottle out of his mouth. Earlier he used to drink 100ml but now I give him 80 to 90ml in two phases. His wieght is at lower side I.e5.3 kgs n he is not gaining proper weight as per his age, but he is active kid. we consulted a pediatrician n he prescribed MIMz drops n ultra D3. for digestion he gave neopeptine drops. Please suggest what i can do for his weight improvement n how to take care of feeding as he is rejecting the feeds.