I wouldn't worry Sounds like he is just really active and burning it off. Also sounds like you are giving him a well balanced out diet too. Good job

You also need to bear in mind genetics, if your families are tall and thinner build then it isn't fair to expect a child to be on the higher percentiles. Plus in order to have a percentile chart some children need to make up the bottom %iles just as they need to be in the top too

My DS1 was 9.68kg and 75.5cms long at 15m and DS2 was 9.4kg 78cms long and HC of 47cms. DS1 was 1545gm and 40.5cm at birth and DS2 was 3090gm and 50cm so that just goes to show that genetics eventually take over despite birthweights too

At 15 months, they were developmentally worlds apart, DS1 hadn't started taking steps yet and DS2 was running, climbing and into everything.

Hope that helps x