What Causes Small White Headed Pimple On Elbow?

My son had a small white headed pimple on his elbow. Now the area around it is swollen--about 4 or 5 inches below the elbow, and about 2 inches above. He went to the doctor today. They were not able to draw any fluid out of the area. They x-rayed it, but didn t find anything that they mentioned to him. He did see what looked like a small area that may have been a bump on the bone. It is very painful and tender to the touch. He has to keep it elevated to minimize the pain.