What Causes Severe Lower Back Pain With Sharp Pain In Leg?

My son is 18 years old and he woke up with severe pain in his lower back with a continuous sharpe pain in his leg. He has had a knot in the center of his back for over a year. The doctor had it x-Ray and said that it appeared to be extra bone on the spine. Right now his lower back feels like it may have fluid on it. What could this be?