Latest development is that he is going to a Coldplay concert and will be flying there.
Coldplay are on a world tour but are not playing in our country, certainly not the city he says he is going to
If you have any influence over him I'd try to prevent him going to this or meeting anyone about it. I'd be worried that someone else will pay the flights, then coerce him into being a drugs mule because he "owes them". Of course there will be no concert on arrival.
He has always been a bit ‘off piste’ and it has been suggested to us that he perhaps has a spectrum disorder. But that is quite in contrast with his childhood where he seems to have been fairly normal and achieved quite well in school
That's not a contradiction at all. Some neurodiverse people don't cope with school. Others can mask while they're there and then it all falls apart at home. Some it suits them, the certainty of the class schedule, the daily routine, the being thrown together with others daily which can hide the fact they have no real friends, obsessive tendencies can be channelled into studying and passing exams. None of this is anything like normal adult life, so the problem may not become apparent until they reach adulthood.
I wonder how many homeless people have undiagnosed MH problems that have resulted in them being on the street?
Lots.
He has had a disagreement with every employer he has worked for.
He has not held down a relationship since I’ve known him
This points towards personality disorder, neurodiversity, learning disabilities maybe. Basically anything that causes him to think in a way that's different to most people's thought processes and to believe that he's right, even when it's obvious to others that he's not. Or that causes him to not be able to cope with things that others find easy to cope with. So someone asking him to do something or expecting something of him, they're assuming it is totally reasonable but he thinks they're an unreasonable arse because he can't cope with it (and don't realise it's not like that for others).
Does he own a property? Live with you? If he has no money he can't be paying rent, unless there's welfare benefits for that? You're buying him food but who's paying for the internet and phone?
You can't force him to get help. Sadly, for some people their problems don't become apparent until they're in trouble with the police multiple times and the pattern of behaviour is officially noticed. Even then, it's unlikely anyone can force them to engage with any help that's offered.