I used to teach reception and don't remember any child not knowing which was their dominant hand when they started. My ds is 4 and a half and regularly uses both hands. Are some kids ambidextrous? Hes not that great at writing despite having good fine motor control whilst doing other stuff (lego/playmobil). I can't help but feel that he will improve when he decided on a hand. It affects his scissor skills too.
Although I suppose being ambidextrous would be good too. Am not worried but just wanted to find out their were many around like him?