How To Treat Fecal Incontinence In Children?

My son is almost 11 years old. He is always pooping his pants and he smells all the time. No Matter how much i show him how to clean himself or resend him back in the washroom to clean up he won t do it. When he was younger he would hold his bowels as long as he could. When asked why he does this he replies by saying he doesn t know. I don t know what the problem is but he he is getting older and the problem isn t improving. I can t figure out if this is mental or physical. Please help with some information.