Suggest Treatment For Knee Effusion

My son fell while running at a fast speed and he tripped on his knees. My son did an x-ray on his knees at the clinic last week and was referred to the orthopedic clinic this coming Mon, 17 Feb 2014. The report says knee effusion, consider fracture. He is on plaster cast now. I was wondering what would his treatment be after he visits the orthopedic. How long does he need to be on cast?