NC for this.
My PILs never did this and I think it's contributed massively to them now having a drug-addicted, mentally unwell, socially isolated and incredibly unhappy 35 year old son, against whom they now have a restraining order in place.
We too have done our time with BIL, as have several branches of the family. He stayed with us for a while (other family members saw it as a glorious opportunity for a fresh start for him, us not so much... v long story there). It didn't work out.
For me, weed in the house was an absolute red line. I said to him he could smoke if he wanted but not on our property and that was that.
In short... I think that giving your DS small objectives at this stage with the threat of turfing him out as an ultimate deterrent is tree best way forward. Say to him that whilst you love him, you're going to work towards helping him to live independently over the next couple of years or so because you believe that's the best thing for everyone.
Seriously if he's still like this at 35 he needs to be doing it in someone else's space and not yours.
So the first objective would be...
Within two weeks you have to be seriously looking for a full time course to study. This means that in two weeks we're going to discuss options around that with you. You don't have to have signed up, but you do have to look.
Or..,
In two weeks, you have to seriously have sorted out your room so that we can help you redecorate.
One of the problems is that when young people go off track, getting back on track seems so overwhelming for them. It's hard to sort out your living arrangements, your income, your social life and your family relationships if things have got behind. Is easier to have a joint and a nap quite frankly.
And then, have a fallback position of 'you can't stay here' timeline in place of say three months that he knows Scot. If he doesn't work towards the easier, practical activities, reserve the right to activate the ' you have three months to get out' clause.
This way it's all under more control, everyone knows what the plan is for if things go off track, there's no need for heightened emotions or anxiety-ridden knee-jerk decision-making at crisis stage because you have a plan that you can follow through.
If he can't get on board with this, then you'll activate your plan, or he'll walk anyway.
Yes he's young, yes things are expensive, but he's not a baby. He's not too good to do lots of hours of low-paid work if need be to keep him stable.
He needs to feel like he has agency, but in manageable chunks, where you're not constantly in conflict with him.
I say all of this because we have had other family members in similar situations too, and I've had to think what I'd do if this happened to my own DC.
Of course he's your son, but you can easily get to the point where you're setting yourself on fire to keep your adult child warm. Please don't do that.