The knee x-ray shows

a small curvilinear bony avulsion adjacent to the lateral tibial plateau implying a Segond fracture.

Segond fracture is a fracture associated with an internal derangement of the knee. A small bony fragment is pulled off the posterior lateral tibial joint line by an avulsion of the lateral joint capsule; this is almost always associated with an ACL tear.

This patient should have his knee immobilized in a splint and referred to an orthopedic specialist.

Reference: Fundamentals of Diagnostic Radiology by Brant and Helms, 4th edition