Jaw dislocations can happen if they strike an object (rock, glass, etc) when going after food. There is not much you can do. The jaw might set itself. If the dislocation is sustained, the fish might have a hard time eating. Keep an eye on the fish for now.
I agree, leave the lion alone for now, don't try to feed it, my black dislocated it's jaw about 2 years ago, it started slamming the jaw against my LR, the jaw reset, it took weeks to get this lion eating again, I had to feed very small pieces of food, and just baby it along.
Good Luck, keep us updated on your lion............
Kaye