If he is happy and healthy and eating well i wouldn't be to concerned.
I also think genetics has a lot to do with it.
My DS3 is 9 and is just over 20kg...he is a scrawny boy but I am 5'6" and 48-49kg so he is very much built like me.
My 2.5yr old is also very petite at not yet 10kg and 87cm...very petite like my XDH's side.