Sympathies, it is hard! I can remember how cut adrift I felt at a similar age even just coming home for the holidays from Uni! It wasn't that I didn't love my parents, but I was used to doing my own thing without a barage of questions and cooking/eating when I wanted etc, plus I missed my friends so much.
My dd has recently moved home after 3 years at uni followed by a year living abroad. She has been pretty busy and away quite a bit, she started her Masters today and is living with us until next September.
We have had a few flash points but we have settled down now, we live in an area with great public transport so we don't have the issues around that (which I would say is probably the worst thing for your ds, feeling stranded and not able to go anywhere without asking for a lift must be awful!) You say you haven't always lived rurally, so have you moved since he was at home? If so he presumably doesn't have any local friends, which could be making him feel very isolated.
When we do have an issue I try and put myself in her shoes, and also remind myself that this is likely the last period of time we will all live together and so try and make it as enjoyable as possible rather than feeling resentful. I also remind myself that it is her home as well as ours, despite it being pretty much just us for the last 4 years. I know I would have hated moving back home after uni and this generation really don't have it easy in that respect, we could get housing benefit and sign on if we couldn't get a job, so we really didn't have to move home. Anyone who did was thought of as a bit odd (rather unfairly, but that's how it was).
So I have cut her some slack, but also had conversations about food (we get food she can cook for herself as we don't know when she will be in/out and I hate food waste, but we have agreed we will all eat together if we are in the house) and tidiness (she is much better in this respect since living abroad). We are financing her as her Masters is very much full time contact hours + many evenings, but in return she is doing some work for DH helping with his business.
DD is lucky in that she has the top floor of our house so has a bedroom, a sitting room/office and a bathroom to herself, so she can choose to keep herself to herself and can have friends over without us all being on top of each other. Would there be any way you could carve out some living space for DS so he has his own space, it's not for ever!
Would it be possible for him to get a moped/scooter to get around on, you can drive those on a learner's permit after an introductory test I think. Or even an E bike depending on quite how rural you are.