welcome to hcm
it could be an onset of knee joint arthritis
although its not that common at the age of 26 his weight can be a contributing factor.
walking does not play a role in it.
consult an ortho.
take an Xray knee joint AP and lateral view.
hot water foment the knee twice a day.
practice static hams and quads exercises
weight reduction is required.
since obesity can lead to various health problems apart from joints
he can do swimming, jogging going for a walk
avoid too much oily and junk food.