He's about the same weight as my little guy who'll be 2 in Feb. Yep he's little....he's just a pocket rocket though. As one mama on the lower end of the weight chart to another, your little guy sounds like he's doing just fine.
Their weight seems to plateau about that age in my experience. DD was a fair bit heavier (and a whole lot taller than DS) at 15kg at 2 years old. She didn't put on any weight until nearly 4 though.
I also found that they had changed the weight charts from when DD was born so on her baby book charts DS is fairly average (the boys ones) but on his own in his book he's on the bottom....just another thing they want us to stress about