Although a little old check the hip and clear it as a potential referral site e.g. slipped epiphysis. If there are local knee signs (e.g. tenderness over fenoraml condyle and pain with loaded knee ROM) then an OCD lesion should be suspected and ruled out given his age and activity it doesn't sound like a loose body yet but it may be the early stages. Beyond that I would throroughly check the hamstrings and associated neural provocation tests as a potential source.