There's a long list of diseases for which "CFS" is just an umbrella term.
For an exact diagnosis, try and persuade the GP to refer him to the specialist doctors at the Lousie Coote centre in London. Unfortunately it's the only such centre in the entire UK (that I'm aware of) and the travelling expenses are a pain if you haven't got a job. You have to visit annually to review/renew the prescription medicines which your local GP just isn't authorised to dispense.
(You can't prescribe a treatment if you're not specialised enough to diagnose the specific variety of CFS in question)
In my case, I caught chicken pox in my twenties, then came the fatigue and the depression came as a response to struggling to get through the working day and not having enough energy to have 'a life' outside of work.