What is the ruling on one who did not know of the obligation to wash the entire body in ghusl for janabah, and are his prayers accepted, and is he considered an infidel because of that, and what must he do?

This person's prayers are invalid due to the absence of the purification requirement. However, this does not make him a disbeliever, but he may be sinful for his negligence in seeking the obligatory knowledge. He must repent, and the majority of scholars oblige him to make up those prayers he performed without purification. So, let him enumerate them and make them up according to his ability. If he is unable to ascertain them with certainty, then let him act based on estimation.