Having said that someone else will be able and willing to his job (which I think is true because of all the reasons that attracted and have kept him doing it), it is not necessarily the case that someone as motivated, with his depth and breadth of knowledge, or as determined to do a job well, would take a job in another sector or organisation.
My point is that he could actually make more of a difference doing a less obvious or 'glamorous' job really well.
Bear with me, as I may sound like I'm arguing against myself now but I'm not. I've worked in the charitable sector of a different profession for years (in which every large NGO and public body has their main office outside London, incidentally) and one thing I've learnt is how much difference a good person can make, in all sorts of jobs.
When I started out as an idealistic youngster, I thought 'what's the point of doing a job that someone else would do, there's only any value in doing something new, that wouldn't happen otherwise'. Then I saw how differently people operate, how much can be developed and achieved by someone performing a role really well, and how useless and disappointing it is when someone isn't, so how dependent supposedly 'normal progress' is on good people.
That might sound like an argument for him staying in his current job but it isn't. There is a massive difference in the attractiveness of different sectors and roles to different types of people, so there will always be driven, impressive, impassioned people ready to do the high profile, frontline stuff - for some sorts of jobs, often the worst paid, entry-level charity ones, the quality and number of applicants is always incredibly high.
For the public sector, grant-giving body or private sector roles though and for jobs requiring certain types or amounts of experience, the recruitment shoe can sometimes be on the other foot and the same people might not apply (I don't know, maybe they fly high, burn out and go off to grow vegetables on a small-holding in Wales instead).
I'm not saying that there aren't really good people working in private sector organisations etc. I am saying that cross-sectoral experience is often valued and that he might be able to make connections and drive a role forward in a way that someone less steeped in his subject might not. So he could actually achieve a lot and see the real difference he's made, from behind a desk.
Of course there's a risk of clash of cultures, frustration, square pegs in round holes and all, so care needs to be taken and a willigness to adapt working style might be necessary but just, don't dismiss other types of organisation.