If the pain is mostly underneath the cap it says that the joint line
pain would be there. Pain at the joint line
is the result of a collateral ligament or meniscus problem (or both) until proven otherwise. Anterior knee pain can also be due to Patello-femoral syndrome.
When did his pain begin, what was he doing at the time, and what were the initial symptoms?
Does he experience any grinding, locking, catching, or giving way of the knee?