Answer:
His diagnosis is probably Illness Anxiety Disorder (formerly known as Hypochondriasis).
Explanation:
Recently, the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) redefined and modified what was previously known as Hypochondriasis into Somatic Symptom Disorder and Illness Anxiety Disorder.
In this case, Evan has probably Illness Anxiety Disorder, Care-seeking type. In contrast with Somatic Symptom Disorder, this focuses on the anxiety caused by the significance or outcome of the presumed illness, not by the somatic signs or felt symptoms.
As directly lifted from DSM-5, in order to diagnose this, the following criteria should be met:
Criteria A: Preoccupation with having or acquiring a serious illness.
Criteria B: Somatic symptoms are not present or if present, are only mild in intensity. If another medical condition is present or there is a high risk for developing a medical condition (e.g., strong family history is present), the preoccupation is clearly excessive or disproportionate.
Criteria C: There is a high level of anxiety about health, and the individual is easily alarmed about personal health status.
Criteria D: The individual performs excessive health-related behaviours (e.g., repeatedly checks his or her body for signs of illness) or exhibits maladaptive avoidance (e.g, avoids doctor appointments and hospitals).
Criteria E: Illness preoccupation has been present for at least 6 months, but the specific illness that is feared may change over that period of time.
Criteria F: The illness-related preoccupation is not better explained by another mental disorder, such as somatic symptom disorder, panic disorder, generalized anxiety disorder, body dysmorphic disorder, obsessive-compulsive disorder, or delusional disorder, somatic type.
