The newest diagnostic criteria can be found at iom.edu/mecfs
Fatigue and impairment There is sufficient evidence that fatigue in ME/CFS (SEID) is profound, not the result of ongoing excessive exertion, and not substantially alleviated by rest. This fatigue must be accompanied by a substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities and persist for more than 6 months. Fatigue, and particularly the impact of illness on function, should be assessed in making a diagnosis of ME/CFS (SEID).
Post-exertional malaise (PEM) PEM is worsening of a patient’s symptoms and function after exposure to physical or cognitive stressors that were normally tolerated before disease onset. Subjective reports of PEM and prolonged recovery are supported by objective evidence in the scientific literature, including failure to normally reproduce exercise test results (2-day cardiopulmonary exercise test) and impaired cognitive function after exertion. There is sufficient evidence that PEM is a primary feature that helps distinguish ME/CFS (SEID) from other conditions.
Unrefreshing sleep Despite the absence of a specific objective alteration in sleep architecture, the data are strong that the complaint of unrefreshing sleep is universal among patients with ME/CFS (SEID) when questions about sleep specifically address this issue. While polysomnography is not required to diagnose ME/CFS (SEID), its use to screen for treatable sleep disorders when indicated is appropriate. Diagnosis of a primary sleep disorder does not rule out a diagnosis of ME/CFS (SEID).
Cognitive impairment Cognitive impairment in ME/CFS (SEID) includes problems with thinking or executive function exacerbated by exertion, effort, or stress or time pressure. There is sufficient evidence that slowed information processing is common in patients with ME/CFS (SEID), and a growing body of evidence shows that it may play a central role in overall neurocognitive impairment associated with the disease (memory impairments, attention deficits, and impaired psychomotor function). Such a deficit may be responsible for disability that results in loss of employment and loss of functional capacity in social environments.
Orthostatic intolerance Orthostatic intolerance is a general term that implies worsening of symptoms upon assuming and maintaining upright posture. Symptoms are improved, although not necessarily abolished, by lying back down or elevating the feet. Sufficient evidence indicates a high prevalence of orthostatic intolerance conditions in ME/CFS (SEID) as measured by objective heart rate and blood pressure abnormalities and physical findings during standing, bedside orthostatic vital signs, head-up tilt testing, or by patient-reported exacerbation of orthostatic symptoms with standing in day-to-day life. These findings indicate that orthostatic intolerance is a common and clinically important finding in ME/CFS (SEID).
Keep in mind that if it does not look something like this, there are lots of other reasons for fatigue. While depressed people do have fatigue, people with a wide variety of other illnesses do also. It's possible to be depressed and have another illness [whether the one discussed here or any other], or for discouragement or low energy to look like depression to an observer. Your brother needs your support. Please listen to him carefully.