What is the ruling on a mother touching her son's bare back below the navel to apply ointment?

A man's 'awrah is from the navel to the knee. It is not permissible for your mother to look at it or touch it, except in cases of extreme necessity, such as if you cannot apply ointment yourself, or if there is no wife or man to apply it for you. Only in this case is it permissible for your mother to do so.