It could be genes that's why your LO is small. However, need not stress about whatever opinion you get from doctor/nurse. As long as baby's healthy. At this point, since baby's 6 months, you can start to give puree, cerelac, porridge besides milk. You can see the progress of the next check up upcoming months.
no need to worry mine at the opposite my 5mo is 9.5 kg ..doctor ask me to go peadiatric but im not going cz he have big size when born and fully breastmilk. as long he is healty active and reach the milestone on their age