It could be an injury with subsequent infection. Is he in quarantine? If not, do try to get into the habit, it really will save you MANY a heartache. If not, treatment (if necessary) is best done in q/t (which does NOT have to be a "fish tank", it can be any watertight, chemically inert container). Feed him fresh foods, soak in Selcon, keep water quality pristine (zero readings on all nitrogenous params), and WATCH. If you're lucky, he'll get over it on his own and it'll be no big deal.
