Injured Knee, Looks Dark Red And Occasionally Bleed. Should I Go To Dermatologist?

my son injured his knee about 3 weeks ago. He pinched it in his room. I thought it created a blood blister so I just let the area be and figured it would go away on its own. Well it hasn t. It doesn t look like a blood blister anylonger however. It is the size of a pea and looks kind of like a growth of some sort. It is dark red in color and occasionaly bleeds . The other day he rubbed his knee on the cooffee table and it seem to lift on one side but hasn t continued to lift. I have placed a protective plsctic like gauze over it to prevent it from catching on anything but just wondering what it could be. I just trying to figure should I go the his PCP or a dermotoligist for this.