PLEASE HELP WITH MY SON

So I have a almost 4yr old son July 18th he will be 4 July 18th, he is so super smart knows all his shapes, colors, counts to 10 BUT HE WONT USE THE BATHROOM he is still in diapers I have tried cheerios, pull ups, being super excited with him, even bribing him smh nothing is working and I brought concern up with doctor and she said when he is ready he will do it himself 😳😳😳😳 I don't want him in diapers in kindergarten... other people say if he poops himself or pee beat his behind idk about that one any insight or help would be appreciated