The way it shrivelled, then fell off in one chunk, when he seems otherwise healthy makes me wonder if he wounded it in a way that damaged blood flow to the rest of the dorsal, causing it to die and fall off. I've seen that happen to fish who had their tail shut in cup lids, for example; the fin will be intact, but after a couple of days it'll shrivell and fall off. Unless it gets worse, happens to his other fins, or refuses to grow back, I'd say just keep an eye on it and keep him nice and clean.