See a doctor soon. Could be something else. Def better to find out early. I have a RN friend and will ask her........ ok, She said check thyroid level as a possibility. She agreed, see a doctor. She also said don't look up condition online.

Start with thyroid a good medical basis.

While at it check for Coeliac disease. This is a type of food intolerance, where your body reacts badly when you eat gluten – a substance found in bread, cakes and cereals. Other symptoms of coeliac disease, apart from tiredness, are diarrhoea, anaemia and weight loss. Can be very mild or severe. Could be a very mild case.

Does he have difficulty sleeping, wakes suddenly at night, snores? Could be sleep apnoea.

Lastly, considering the loss of sexual desire, could be depression or anxiety/stress related fatigue.

Simpy go see a doctor.

Of course after making sure it is not a serious health issue, I would invite him to cultivate his sexual energy/life force through tantric practices. Massaging the penis and balls can be very beneficial, avoiding ejaculation, just raising energy up. Using ice on balls and stretching them gently can help for testosterone production.