He can determine your condition not stable enough to perform your job safely.
There is case law regarding licensure. I know of many R.N.'s who work in hospitals despite having a history of epilepsy. The practice of nursing may be restricted (as for example a nurse anesthesiologist), but it is not up to your employer (in the United States) to "pull" your R.N. credential.
Do not delay seeking an evaluation becuase of this worry. All things being equal, I think that most neurologists would try to work something out to enable you to continue in your profession.