Worried! Is my son small?

My boy had his 6 months vaccinations today and they weighed and did his measurements. He weighs only 6.2 kgs and is 61.5 cms long.

The doctor said that his falling below the growth chart and wants us to come back in 2 weeks for a check up.

My son is EBF and was born 2 weeks early only weighing in at only 2.66 kgs so he had more than doubled his birth weight.

He has regular amounts of wet nappies and poops and is overall happy and met all his milestones. I started him on rice cereal at 4 months and now his on purée. He gained weight really fast from birth up until about 3 months and ever since weight gain has slowed down. Should I be concern? Here is a pic of my happy chap!