James:
My intial guess is what Terry mentioned, in that the lion had stuck something w/ its jaw, my second choice is a goiter or tumor. Unfortunately you described the situation which often causes one of the bigger problems for lionfish. A nutritional deficiency and freeze dried krill IMO is the biggest culprit.
Since its been a few days
Will your lion eat at all now?
if so, please go to the grocerry store and buy gulf shrimp w/ shell on, crabmeat, scallops, squid. Also get silversides, and any other marine fish. Rotate thru these foods and suplment w/ selcon and you might reduce this problem in the future.
I would give him somemore time and see if he turns around, but you really have to feed him much better. The whole tank for that matter
frank