Most of the Masters courses run by the OU just require a Bachelor's degree, sometimes with Honours. I just looked at about 10 different subjects and the only one that a 3rd wouldn't qualify for was Mathematics. Of course, in the case of the OP who's son wants to work in a lab, distance learning maybe not the best option. For people like me with Honours degrees at a low classification, the OU is a great way of topping up to postgraduate level. There's a little bit of smoke-and-mirrors possible then, in that (as highlighted by posts in this thread), there is an assumption by some people that anyone with a Masters must have got at least a 2nd, so you can kinda obfuscate the poor result to some extent.
There's also night study if you're in London, but the OU serves its purpose for everything else (London International Programme -- UoL's distance learning setup -- is really not for people who struggle with organising their workload, as it is predominantly based around exams).
Even that restriction for Maths is fairly recent, from what I can tell. At permitting a 3rd, their completion rates had statistically plummeted to an unsustainable level. The department was already under pressure to steady its profile out and the new fees regime brought the matter to the fore.
Looking at how they've been expanding content on earlier courses, and reviewing the postgrad modules, it's safe to say they'll be penalising hobbyists in favour of younger students, with at least some A levels to their names, going forward. Expect a heavier emphasis on more cite-able applications and stats, too. Whether this attempt to massage standards succeeds in making the OU rise on par with Bath, Heriot-Watt and Warwick, just to take a few institutions of a similar age as unis, with a good maths department, is yet to be seen. Really depends on what they wish to specialise in and what staff they plan to hire; the latter being their main problem: too small, and, unlike outliers like Dundee or Pompey, too generalist (particularly if you include the educators, historians and all the theoretical physics bods filling out the ranks).
/end mini rant
