Small baby?

I just went for my LO 6 months check up and his weight is only at 6.2kg while the chart at his health booklet is showing that he should at least be at 7.5kg. He’s been drinking 120ml every 3-4 hours and having solids 1 time a day but his weight just can’t seem to increase. At this point I’m not sure if he’s just a petite baby or he’s not consuming enough? Anyone here having the same problem :(