You do realise he's not going to move out of his own accord? He's stormed off in a huff that's all. He's not going to voluntarily spend less on weed so he can pay rent, especially not when he knows he can bully you and your husband into getting his own way. You talk about seeing the error of his ways, but with no negative consequences for his bad behaviour it hasn't been an "error" for him, it's worked very well! Change the locks before he comes back from whichever mates floor he's sleeping on.
Would you give him money for a room to get him started.
No. You may as well be giving him money for drugs.
Do I let him sort a room himself or help him pay the deposit etc.?
It's not going to be a case of "letting" him sort a room for himself, more forcing him to. Otherwise he'll still be lazing around in your house, smoking weed and surrounded by the detritus of his failed life ten years later. Don't pay the deposit, he has plenty of money he's just spending it on drugs.
what if he asks to borrow it.
I can see how you ended up in this situation! Just. Say. No. And keep saying it whatever "reasons" he comes up with for why he should have it. He doesn't need to borrow money, he can stop smoking weed for a few weeks to raise the necessary funds. Stop paying his car insurance too. If he can't afford it he'll have to use public transport or cycle. Personally I'd prefer someone who's stoned to not be on the road anyway, if he gives up driving it'll be doing society a favour. Alternatively he can do himself a favour and give up drugs to afford insurance. It's not rocket science is it.
Having to make some adult choices could be the making of him. Good luck with the eviction. Don't let him store all his junk in your house until he's 45 either. Moving out means taking it all with him or disposing of it.