Hi, My Son Is 6 Months Old. But His Weight

Hi, my son is 6 months old. But his weight is below normal range. He weighs just 5.8 kg. His birth weight was 2.9 kg.
I feed him along with formula milk 60 ml. Once or twice a day..
Now after 6 months I have also started giving him purred fruits veggies curd & Apple Orange juice. Sometimes cerelac. But still he is not gaining much. Everybody says he looks no more than a 4 month old baby. Please suggest. I'm too worried..