Hi there, if your son has been acting normally, and has not been throwing up, this is likely just a bruise and swelling under the skin and not cause for concern. (The swelling is sometimes referred to as a "goose egg"). If he is acting like his normal self, I would not be overly alarmed, and there should be no reason not to let him sleep. On the other hand, if he is crying inconsolably, not acting like his normal self, begins to vomit, is sleepier than usual (not waking up normally when you rouse him), or you are still concerned for any reason at all, then I would take him to an emergency room without delay for imaging. The first step would likely be a CT scan of the brain. I hope this is helpful and reassuring for you!