I didn't think it was that big a bang, and he didn't cry uncontrollably, however, since hitting his forehead (against a table) he has been sleepy and almost fell asleep on my lap. I stirred him awake and he's now watching vids on youtube while I type. He hasn't vomited or anything like that.
He hasn't napped today (he didn't want to - he doesn't usually nap these days) so of course it could just be natural tiredness in the heatwave. But I don't know whether I should be overcautious as I've never had to deal with this before. I'd appreciate any advice!