If there is no physical cause for any of his symptoms then it's likely that Xerxes has some form of mental health disorder which is causing him to experience various ailments. Mental health disorders can cause very real physical symptoms of illnesses in people and it is important that medical professionals are aware of this. It is also important that medical professionals do not write physical ailments off as just emotional health issues as this isn't always the case if they can't find any physical reason to a patients problems. It works both ways.
Xerxes is most likely suffering from FUNCTIONAL NEUROLOGICAL SYMPTOM DISORDER. This is a type of disorder in which the patient experience various symptoms but the systems of the patient is perfectly normal but function incorrectly.
