Some thoughts:
A general degree in a traditional -- hence easily recognisable -- subject (at random: Pure/Applied Maths, History, Physics, Philosophy, English Literature, Law, Computer Science, Engineering, Medicine, among others) is usually done to either go into a profession, work out what one wants to do in life or pursue a passion.
A Third therefore presents a problem, albeit not an intractable one, for all three routes out of undergraduate education. It's hard to give detailed advice without knowing the whole history of how we got here, your son and what he wants to do, really; nonetheless, let's put a few options on the table and avoid panic. Although be aware that no route will entirely erase his prior academic history, and he will need to develop a compelling, evidence-backed argument for potentially tough interviewers and application forms, regardless of where he goes. (Lying about your qualifications or not being able to justify and explain results you got is a big no-no in both academia and industry. But with wit and charm 'failure' can be handled rather better than people realise.)
Take a short break and plunge back into academia (having brushed up on the trouble areas):
If he's passionate enough and it was largely bad luck and personal circumstances that limited his potential, it's perfectly feasible to come back from a Third. Although I would not advise trying to salvage too much via retakes of exams, years or coursework over the summer: these are often capped, and are of use only in either getting a qualification or a borderline case to perhaps salvage a moderately higher pass; depends on the subject and university policy, of course.
So how to dodge the Third here? Well, HE is a consumer industry, more or less, at this point of his education, and so there are always other products he can have a look at, namely postgraduate diplomas or masters which integrate them (2 years). Postgraduate modules are on average harder and there's at least one more piece of extensive independent work he will have to complete (project, dissertation, thesis, etc.) to get an actual masters. Then progress on from there.
The caveat being that it's more time, more cash and things get less and less structured and much harder later on. He might be forced to take his PgDip course at a fairly lenient institution which may be miles away from home/his current city of residence, jumping once again to get a Masters at a place with a bit more prestige or employer reputation (as a fallback) for the course. It will be a stressful slog. On top of that, the chances of a steady academic career in the UK are low; starting off a third they are non-zero but become vanishingly small.
Pick up any job, study part time and retry for a career with a better result:
A middle-of-the-road and fairly pragmatic approach to take. He can earn some cash, recover a bit and pick up study in the evenings or via distance learning. The transfer of academic credits from his better modules is also possible for the same or related subject areas (so he'll have less to do again overall); The Open University, for example, is reasonably flexible in this regard.
It's possible to progress all the way up to a PhD via part-time learning, whilst keeping your options grounded and open. You never know, he might also stumble into an area of work/career which will have progression routes that are not reliant so much on academics or offer vocational training/independent certification. If he's a good talker, Sales is a decent enough leg up.
Of course, going into work off the back of what can be construed as a personal failure can have a psychological effect on him, and he can equally end up in a situation which bores him to death and destroys any last shred of creativity and original thinking he has left. Part-time, distance and evening study is also not for everyone: working, earning and sacrificing your free time for a qualification is daunting and requires flexibility, planning and determination to succeed come what may.
Still, a good degree in many cases is a good degree, and grad schemes will open up to him again in the end.
Do a 'conversion Diploma/Masters' course and exit with that:
Is he in the right subject? Most young people aren't! Say he started in Maths and hated it but always loved computers, programming and creative problem-solving regardless; why not try Computer Science? But as with the first option, same caveats apply. Plus he might dislike or struggle with how the subject he studied connects to and is manifested in industry. But having a Masters is a notch up on his CV, and sidestepping into a different career with some work experience with a stronger terminal qualification at one's back is easier than without.
He we'll also need to get some independent career advice and counselling to make this really work well for him.
Choose a profession with a separate qualifications route:
Accountancy
Risk Management
Skilled Trades via an adult apprenticeship
Certified IT Tech/Developer/Architect/Database Administrator
and the like. This route can either work out cheaper and more fulfilling or an even greater money sink (without employer support) and disappointment, as some careers that are in demand and pay well are almost unbearable to handle without the right mindset or willing to do anything for a stable pay cheque. Initially finding an employer that will sponsor your son or offer him any training may be difficult, but the beauty of independent and vocational qualification tracks is that you can go up partway yourself and show an employer you're a good investment long-term.
