Well not having two boy I cant speck for myself but a GF of mine who has two said her boys are too different sizes and no two men are created equal... So maybe he just going to be a wee bit smaller than his brother.... I would worry about it to much compare to DS size his is tinny winny but I am sure it too will grow ...

The good thing about boys is no one but himself and his Partner ( when he is older enough ) will know his size even you will never know what size he becomes...

Unlike two girls if one has boobs and the other doesnt everyone knows...