My 10 year old

My son who is now 10 year old still poop on his pant and will not clean himself until he is smelling and somebody tell him to clean. We can smell him and will still deny that he dirty and will continue to play. I'm afraid that he will get tease in school. Plus it really smell bad and very unsanitary. I'm divorce from his dad. He have a stepdad that  treat him his own son. But this is creating problem cause he won't do any thing unless he is told to clean him self. What do we do. Rewards,punishment not working at all. We need help.