Knee pain from small object after a fall

My son "16" was playing in a pre-season Varsity game and was tripped up hitting the floor. He didn't have any pain to speak of and continued to play. The next day his knee really hurt and there was something moving ,about the size of a M&M, it could move from left to right moving under the knee cap. When the knee bent with the object not in sight there was a lot of pain "kinda locking up the knee" Xray showed nothing so it was not bone. The doctor could feel it but could'nt figure it out for sure. My son walks fine today with little pain and the little object can not be found? Has anyone ever heard of this type of incident? I'm afraid to release him to practice without knowing what's happened to his knee. wink@proaxis.com