My eight year old still pees and poops on himself

Patient: My 8 year old son has never been potty trained. He will still pee and poop on himself if he is not sent. We have see neurologist, had mri’s of brain and spine for tethering. We have seen a urologist and done bio feedback and nothing abnormal was found. We have tried charts, rewards, punishments and are even trying hypnosis. We have also tried therapy with no luck. Nothing is working. On occasions he will go on his own so I feel he knows the feeling but he chooses not to go. It is all the time, at school, at home, with friends. I am unable to find any information anywhere. I am wondering if this is something anyone has ever come across. We are at our wits end.