Oh gosh OP, I can't imagine how horrible it must be watching him throw away a golden opportunity. And FWIW, I do think it's relevant that she's not going into higher education. Rightly or wrongly, once he starts mixing with other undergrads, she may well not fit in, or seem rather parochial.
Most LTRs don't make it past the first term. The one couple I went to uni with who did make it to 3rd year were utterly toxic (she had given up and he Durham so they could both attend Edinburgh, so hardly slumming it academically) because of the resentment that built up.
I wonder whether looking at the long term might help? Not in a 'she won't let you' sort of way, but more 'if you got this degree, what would you want to do?'; 'where might you do that?' etc. because it may well be that he's not really thought it through.
I disagree with some upthread though. DP and I met at uni. My job would be significantly more lucrative in London. He is well know in his industry and has been headhunted for countries where it would be very difficult (or impossible) for me to work, but I am main breadwinner here. We met at 21 and have been compromising ever since. If either of us had an absolute passion to follow an opportunity we'd do it, but we don't, so we stay where we are. Separately we'd probably earn a lot more, but we're not love's young dream and do analyse our opportunities (spreadsheets!). Also if one of use had a burning desire to follow a dream, we'd both do our utmost to make that happen.
Can your DS map out the pros and cons of the choice (in a spreadsheet maybe) in terms of short term; educational attainment; post-uni goals; long term? Setting it down on paper might help without focussing too much on GF, UNI, DF or whatever else.