Pain sensation at base of feet

Hi,

My grandfather has been experiencing burning pain sensation at the base of his feet for at least 5 months now. His 88 years of age and doesn't really have any other medical issues. He experiences these pains (like a burning sensation only when he walks and its constant pain). If he is lying down and his feet are elevated then the pain goes away. He has seen a specialist which has completed a series of nerve conductor tests and blood tests and they all come back clear. Now his on medication which doesn't relieve the pain and only gives him side effects. what could be causing this?