What Causes Popping And Swollen Knee In A Child?

my 12 year old son has been complaining of knee popping for about a month. he does sports but no one incident caused this. slight swelling. pain but not enough that he cant walk on it. looks red in the spot that he feels the popping. we are seeing an ortho in 2 weeks but I was hoping for some suggestions on what may be the problem. red where popping is upper right side