What Causes Bumps In Arm,knee,buttocks And Ankle?

My 9 year old son has pimple like bumps on his body. They started out as whiteheads but now they look like they have popped. It's raised around the head and red. He has one on his inner arm, one on his knee, one on his ankle and 6 on his buttocks and they all showed up at the same time. Not itchy but painful to the touch.