Gosh, I think a lot of posters are identifying very, very strongly with the 'dull' gf - possibly because of being disapproved of by their own ILs?
OP, absolutely your DH needs to butt out right now - quite apart from it being none of his business, having persuaded your son not to turn down the Cambridge place, a tactical retreat is much the best option now.
Plus you can't tell any teenager that their current squeeze is not 'the one' (even if you believe in that kind of one true soulmate claptrap) - it's likely the son has only the most cursory romantic history before this GF, and while as older adults we've most of us had many relationships and are able to see relationships as not necessarily forever, it's too much to ask of someone of that age. He's presumably not thinking of this as some kind of 'starter' relationship, even if it turns out to be that.
Plus, it's possible (albeit unlikely) he will spend his life with this girl - my DH and I got together aged 19 and are still very happily together in our 40s. Having said that, the reason our relationship worked and continues to work is that we were both ambitious WC youngsters, the first of our families to go to university, and we were very clear on never getting in one another's way, or stopping one another doing anything. So we both lived and studied and worked in different countries at times, and never limited ourselves for the other's sake.
Your son's relationship is different, if the GF is acting as a sort of brake on your son's ambition or freedom - did she try to persuade him not to take up the Cambridge place, or was it simply that he wanted to stay local for university because of her? (Not that it particularly matters - the effect is the same.)
But you're not wrong to worry about the longterm impact of a relationship at eighteen that stops your child doing things they would otherwise have done. I can think of three examples from my immediate circle, but will tell only one.
My goddaughter, immediately before she started university in another city, got a boyfriend at home. When it became clear early that her flatmates in her student accommodation wouldn't put up with the boyfriend (who was not working at the time) living there semi-permanently, Goddaughter began to go home every single weekend to see him the second her final lab/lecture was over, and fell behind on work, because he (lovely but unambitious, live-for-the-moment type who saw work as something that funded your drinking) thought she was a weird swot to be working on assignments at the weekend, rather than clubbing.
To cut a long story short, she never settled at all at university, because she was either on the phone to him every evening for hours, or trekking back to see him at every possible opportunity. She made no friends, and ended up dropping out, having never given either her course or the university a chance, because her loyalties and mind were elsewhere.
Needless to say, the relationship ended within a few weeks of her dropping out and going home, and she admitted she had screwed up and blown a lot of money for a relationship she couldn't see past.
But there is very little you can do to avoid that kind of situation, realistically. Certainly not telling your teenage son he has made a 'lousy' choice of girlfriend. All I'm saying is you're not wrong to worry, but your options are strictly limited, and most of them will risk making the relationship into a star-crossed lovers situation.