Most important is we all recognize that are all concerned parents with different methods. As my wife and I say, it's ok if our methods are a bit different, just so log as our results and goals are the same.
For me, goose egg doesn't always equal hospital visit. For me, I'd keep a close eye on him, watch for mood changes, dilated pupils, lethargy, etc. then again, that wasn't medical advice, best to use your best judgement.
Now the big question - does this tend to be a regular occurrence at this place? Not necessarily with your kid always, but does somebody in the population tend to frequently exhibit injuries like that? I always keep my eyes open for possible signs of child abuse not just my kids, but their friends, my students, kids at the park, kids at the store. I'm sure it was innocent, but it pays to always keep a watchful eye...