OP is it possible that you subconsciously adopt a negative approach to things? By that I mean when your DS tells you news/plans etc... do you have a tendency to focus on the negative and/or what could go wrong? You may not even be aware of doing it - many people aren't. Maybe something to think about.
The reason I ask is that my DM was much like that and as a result, I would always make plans, do my own research, and make sure things were decided and a "done deal" before I told her anything.
You talk about your DS not knowing he will have to pay bills etc... well, as he has not been very communicative with you the last few months, how do you know he and his girlfriend haven't already looked into and discussed this type of thing? As he is living with his GF and her parents, they are surely aware of the intended move, and I would expect have likely been party to at least some conversations and planning. We don't know either way but don't just assume that because he hasn't had a conversation with you about something, he hasn't considered it or isn't aware of it.
Either way, he is a grown man and he will find his way. He is not the first and certainly won't be the last young man to venture off into the world and have to learn a few things the hard way. It's part of growing up and learning about life. Sure, they will probably find it tight for money etc... but so do millions of other people in their early 20's who are making their way in the world for the first time. Many of us have experienced living on beans on toast and lived to tell the tale!
Brighton is a great place for young people. I'm certain he won't have too much difficulty finding a job and he may well meet people that open his eyes up to career interests and opportunities. He may end up doing some further studying himself or whatever - the point is that often, moving somewhere new, meeting new people and broadening one's horizons can be the making of someone and it is how people often discover their path in life.
I'm sure there will be hiccups along the way, and he and his GF may or may not stay together, but ultimately, something like that is a win-win for him because it's life experience and he will learn and grow from it regardless of how the immediate plans work out.
As long as he knows he always has a loving home to return to if he needs it, then wish him well and be excited for him.