Taking xanax for any length of time can result in severe withdrawal symptoms. The symptoms that you describe are identical to some self-reports of xanax withdrawal that I have read. Your dad needs medical attention immediately, and a supervised withdrawal if at all possible, as withdrawal can be very severe. This post is very old, however I felt it necessary to answer the question in full so that others who find this post may refer to my response.
Withdrawal from a drug such as xanax can temporarily cause a more severe version of the problems that it treats (primarily anxiety). Consequently, difficult functioning/sleeping/hallucinations/seizures/bodily pain can all result from withdrawal. I am pretty certain that what you are describing is withdrawal.
On the other hand, General anesthetic can also cause depression/nightmares as well as confusion in some patients, so either way medical attention is strongly advised.
Hi~ Please notify his doctor about this. No sense in guessing...he needs to be evaluated.
Hope he's okay...