If he is unable to eat I would put him out of his misery. Lockjaw seems to be something predatory fish, particularly marines, are more likely to get so I doubt it is the case with your neon. Some fish, like goldfish, have a tendency to swallow stones with the occasional one getting stuck. Again, I doubt the neon did this.
It is possible he whacked into something and damaged his jaw, otherwise I can't think of anything else being the cause. Can't suggest a cure either sorry.