I think in reality a 20 year in a poorly paid job but a home and friends established where you are now is an extremely challenging prospect, he isn’t an established adult by any means. Most newly adult children are supported by parents at university, and come home regularly.
I wouldn’t want to be 300 miles from my child under any circumstances. That is a choice you are making, but you can’t blame him when it impacts your relationship with him, and your relationship becomes cold and distant.
By default you are withdrawing support, and choosing money and prospects over a relationship with your child. It depends how important he is to you, and how much it means to have a close bond with him. Or not.
I would say this will definitely be a defining choice you will be making, and your doubts and concerns are valid.
As a minimum I would be helping him financially to secure accommodation, until he is in a stronger position to support himself fully. Or he could end up homeless.
It is a choice you can make, because he is newly adult, but I would weigh up what it might cost you and him in the longer term.