Lot of bollocks on this thread.
Firstly, Cambridge doesn't 'open doors'/'provide unique opportunity'- going to university, any university, provides access to certain career paths that aren't available to non-graduates, but as someone with a Cambridge degree who is worse off than most school friends who went to 'normal' universities, I can confirm it does fuck all for your 'networking' or opportunities unless you are already the kind of sleazy private school type who would already be handed things on a plate. If anything, there is some reverse-discrimination when job seeking at public sector or other universities, in that they think people who went to Cambridge 'had it easy' and should be working in the city, even if you go there from a poor background, so you end up not only without the networking as there is no way you'll be accepted by Eton wankers anyway, but also excluded from your own background as people think you're suddenly posh now (and like to go 'ha look a clever person is poor, proves we were right never to do homework' etc). All I got from Cambridge (that I couldn't have from any other uni) was chronic clinical depression and frustration at the UK's failure to address social inequality.
Secondly, we have no idea from this that the girlfriend is stupid or 'holding back' the OP's son. She might be lovely and bright, just not keen on university. I wouldn't necessarily encourage my children to go to uni at all, unless it was for a specific career path. I know people who earn double what I do now, having left school with just A levels. Admittedly they were lucky in that there was less demand for degrees-for-everything from inane employers than there is now, but my job, for example, doesn't require the level of qualification that I have. I'd prioritise mental wellbeing above university any time. I really feel frustrated by the lack of decent careers advice for young people, to enable them to make a sensible and informed decision about whether uni is right for them, and what careers require what academic/vocational pathways. I'd certainly have been better going for a more vocational degree at a more practical and socially accepting university.
OP, I think your husband is a delusional dickhead, sorry.