What Causes Difficulty In Bending Knees In A Child?

I have taken my child to the fifth doctor in four years about what they say is severs disease and osgood schlotters. He continues to have trouble, running, walking, and climbing stairs. He plays on a travel soccer team and is now unable to keep up with others because his knees don t bend as other childrens. This has been going on for four years.......I have not read anywhere where this lasts this long.....what else could be going on??? The doctor today took exrays and continued to give me the same story....Each one has treated him differently, advil, casts, physical therapy, nothing....What should my next step be? Is there a specialists that I should take hime to????