Hi OP
How long has he had symptoms?
What triggered the CFS?...virus, bacterial infection, insect bite, tick bite , head injury, chemical exposure etc, etc.
Did he play contact sports e.g. rugby, football, hit his head, suffered whiplash?! Fallen off his bike. Any head injury at all during childhood can lead to problems with hormone imbalance and take years to manifest themselves.
Just some things for you to think about how all this may have started for him.
The main thing to do at the moment is for him to rest as much as possible because exertion will just worsen the condition.
The problem with ME/CFS is that most people with it try to fight their symptoms to try and force them selves to get well and use mind over matter which does not work!
How bad are his symptoms?....Severe? where he cannot get out of bed at all or does he have good days followed by bad days in which case.....
He needs to pace himself.
I was diagnosed with CFS a good few years ago and it took years to find out that the cause of the CFS was that I had damaged my pituitary gland due to a significant head injury.
I have Hypopituitarism. Which means that I have hypothyroidism and severe adult growth hormone deficiency among other issues.
No one was more surprised than me when I found this out(except my GP who had never had a patient with this condition caused by a head injury!).
I was fortunate to get to see and Endocrinologist that knew exactly what he was looking for straightaway. So I did not get told to go home and deal with as I had been told many times before by GPs.
If you want to google CFS and head injury you will find a number of articles about it.
Hope this helps