Is It Advisable To Start Semi Solid Food For An Infant?

Dear doctor, I delivered my second child via csection...I was prescribed L arginine granule and whey protein during pregnancy as baby was not putting on much weight.. Initally he had lost 600 gms after birth then I started with BF and bottole feed both..This month I din t go for any vaccination but he has only put on 200 gms... should I start some semi solid for him n he is taking around 150 ml 4-5 times a d ay. Thanks in advance