GF is still very young. They both are. She may mature and broaden her horizons and want to explore and even feel bad about holding your son back at this stage. But she may not - and for her that's completely legitimate. Not everyone has to want to backpack across the Gobi desert afterall, or even leave their hometown.
But maybe also plant the thought with your son that the issue may not just be which university town but where he spends the rest of his life. Does he want it to be local or does he want or think he may want a wider set of options? At this stage it is, or hopefully it is, about having options and not getting locked into final decisions.
(BTW if GF doesn't want university or to leave her home town or be far from Mum, does she want to start her own family early? Is that something your son would like ...maybe he should think about that too?)
I could imagine GF being scared to lose him to another "world" and yes in an ideal world she would want him to make the most of any chance he has and not be restrictive - but she may just be too scared and young to feel that yet. Is there any mileage in helping her see Cambrigde as a pretty and romantic place she will be able to spend time with your son, where he can romance her and treat her? May balls, punting down the Cam, or whatever else might float her boat? And long term maybe a nicer house in her current local town - ie whatever makes sense or could be attractive in her terms. If I were the GF I doubt I'd be convinced, but I don't think it would hurt to try and present the Cambridge option as positively for her as possible.
The suggestion of at least trying out a year at Cambrigde and then switching if it still feels like the best option is a a really good one I think - for the GF there is a time horizon she might be able to live with and there could be enough time for everything to settle. The risk is that your son is so distracted that he doesn't do enough work...
Also someone outside the family talking to your son too...it helps take the emotions out of it. Better still if it is someone who has stories of long distance relationships doing well.
Actually, now I come to think of it, I do know of a now long-married and happy couple where it did work out. Slightly different circumstances in that the young woman went away to university herself and almost failed her own degree in her attempts to see more of her BF but at heart they were a well matched and committed couple and it worked out.
Overall, though, if there is any hope of deferral that is probably the best option.