I'm supposed to be paying £3k towards car costs
Please don't help him get on the road OP. The world doesn't need another drunk driver. Don't forget he'll still be over the limit the morning after the night before. You already know he's not a considerate person because of all the things he could have done about whatever is the reason he drinks, he chose drinking. Every day, he chooses not to get help for his out of control drinking despite the effects it has on himself and those around him. He prefers to remain a drunk rather than seek help to stop abusing his own mother. He is not going to choose to keep others safe by only driving sober.
want to tell him he has one month to engage with some sort of help, or he has to move out, but I know if it came to it, i couldn't. I did already have him stay with his dad a while back for something he did when drunk, (that was more a boundary he crossed after a few drunken weekends and I just snapped) I don't think his dad would tolerate it again. Now he also thinks I should just tell him to find somewhere else to live. I'd be worried sick about what he was doing every day then because he'd see it as rejection.
His dad can make his own decisions, you can't decide your son lives there.
Son needs to be rejected for this behaviour. Otherwise you're condoning it and giving him the message it's ok to do it. You'll just have to explain clearly it's the behaviour and not him that you're rejecting. He takes it however he takes it. You're not responsible for his emotions OP and you can't live your whole life pandering to them.
You can't manage your anxiety over his life choices by trying to manage (ie control) him, it won't work. Ultimatums to get help "or else" won't result in him engaging fully with help and getting sober. An addict can't change for someone else they have to want to do it for themselves.
Step off the path you're on and get your life back. Spend the car money on therapy for yourself, shore yourself up so you can support him if the time comes that he wants it.
The thing is, when sober, he is great.
The thing is, he isn't sober. That's not who he is now. However much you want him to be. You can't will it to happen. You can't love him into sobriety. Only he can get sober, by himself. Nobody can do it for him.
You need to face reality OP. Look up the Cycle Of Abuse, every time they get away with something it'll be something worse happen next time. Doesn't matter that it's due to alcohol, doesn't matter that he's not your partner. It's the same cycle. You'll walk on eggshells constantly, waiting for the next time he erupts, minimising yourself until you're no longer you just to try to prevent it happening. It won't even work. Every crumb of not-nastiness will start to take on a golden hue and you'll start to view ordinary behaviour as something exceptionally wonderful, such is the relief of a moment in which you're not being abused. By this point you can't even tell how bad it is any more, you're in so deep, because your mind has altered itself to enable you to survive your existence. There's only two ways out of an abusive relationship OP. Either in a coffin at some point (whether that's at their hands or not), after being utterly destroyed by them your entire life, or you walk away and live.
You can't save him. You can save yourself.