Sorry you are having all this worry. 

My husband and I have both, at different times, experienced pain behind a kneecap. It wasn't constant, but felt very sharp and worse in certain positions, particularly lying in bed. In our cases, it was due to meniscal tear caused by sports injuries.  My husband had an arthroscopy and repair after about 18 months. I left mine and it gradually got better on its own but did take well over a year.  

I'm not medically trained and obviously can't tell what your son's problem is but I hope this might relieve your anxiety somewhat. Sport injuries are very common (does he play football?) and bone cancer is extremely rare.