Husband has pain in centre of knee radiating down leg. age 68. has had pain in side of knee previously but not to this extent. painful to walk. ?

See a doctor: Pain is your body telling you something is wrong. He needs to be seen and probably x-rayed. Call his primary care provider who may recommend an orthopedic surgeon ( they do things other than surgery) or take him to an urgent care clinic. Good luck.

Varies: Multiple potential causes, I would not hesitate to be seen. Possibilities can include a sciatica from an irritated nerve in the back, a Tendonitis, a blood clot,among multiple other possibilities. A detailed exam would go a long way in determining the diagnosis.