Suggest Treatment For Large Dark Bruise On Forearm

Hello. I noticed a fairly large, dark bruise with a hard white centre, on my 5 year olds forearm a few days ago. The bruise is about the size of a toonie, with the hard white centre about the size of a penny. He says it doesn t hurt, and he doesn t know how he got it. Today (a few days later) it is still just as dark, and the hard white part is not going away. Do you think this is something to be concerned about? Thank you!