what medicine does he take, theres a two conditions that come to mind juvinelle artritis and Burthsitis (sp) a friend of mine ( many moons ago) had burthsitis in hihg school her knees ached so bad shed cry at times, the doctor to see for the two above conditions is a Rhemotologist, thats a specialist who deals with the above illlnesses and more that are similar, they are treatable but not cureable, they are not life or limb threatining so dont panic.
How old is your friend? Does he see a Chiropractor? What is his weight? Does he exercise/play sports/stretch at all? What type of meds is he on (like muscle relaxers, pain killers...)?