Zaid Alsalami, Shaykh Dr Zaid Alsalami is an Iraqi born scholar, raised in Australia. He obtained a BA from Al-Mustafa University, Qom, and an MA from the Islamic College in London. He also obtained a PhD from... Answered 10 months ago

Bismihi ta'ala

It would not only be permitted to massage your mother's feet, but indeed a blessing and an honour, as you will be bringing comfort and ease to your mother. 

As for legs being 'exposed', to the son, there is no problem with that, as long as of course the mother's private parts are covered. 

And Allah knows best.