Very good. I know that various drugs are marketed under different brand names in different countries, but i could not find any listing for the spelling that you listed in the original question (eyzoir). However, the Ezovir (famciclovir) is in the same class as the acyclovir.
The most likely cause of this constellation of symptoms is that there has been an acute illness that has caused the diarrhea and associated symptoms, which then also precipitated the shingles outbreak. While some episodes of shingles can occur spontaneously, there are also some episodes of shingles that are precipitated by an acute illness, particularly an acute infection.
The most common cause of diarrhea would be a viral
infection, and viral infections can commonly have minor rashes and muscle aches. However, a viral infection of the gut would usually be improving by this point of time. Consequently, the next level of concern would be a bacterial or a parasitic infection of the gut. The usual method for diagnosis of a bacterial infection of the gut is a culture of the stool. There also are stool tests for common parasitic infections, but these are harder to detect, so it is recommended to repeat the test up to three times before relying on a negative test. A CBC also may be suggestive of what type of infection is present. You said that stool tests are pending, and this may clarify the diagnosis, although further stool tests may be needed. If these tests do not clarify the cause of the symptoms, then it may require an evaluation by a Gastroenterologist.
It sounds like he has been tested for the infections that can cause burning with urination, with the exception of a prostate infection, which would usually be diagnosed by physical examination.
While awaiting the stool tests, it may help to try to replenish the good germs in the gut, such as by eating yogurt with an active culture or a probiotic supplement. It also would be important to be drinking plenty of fluids to maintain hydration, offsetting the fluid loss from the diarrhea. Simplifying the diet to a clear liquid diet also can help ease the diarrhea.
If you have any further questions, please let me know.