How to relieve pain from bruised toe caused by door slam?

I was coming in from outside and my little nephew was without shoes from the inside really close to the door. I opened the door with a fast swing and I hit his little foot. His little middle toe was instantly purple... I put cold water for a while and he is now running outside as if nothing happened. His little toe is so purple, I don't know if that's dangerous our not.. is there anything to make it go away? and at times he says "owie owie owie" and holds his foot, but then goes right back to playing.

He's only 2 years old and I'm scared. :(
How to relieve pain from bruised toe caused by door slam?
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