I'm very sorry to read about your and your son's troubles. It must be heartbreaking for you.
The brutal reality currently is that your son is sleeping on a mates floor and most likely drug dealing for money. I say that because of your updates that your son has been arrested with large amounts of canabis and you have paid thousands of pounds to dealers. If he isn't drug dealing now he likely will be soon. He has no means to support himself and can't claim benefits due to his savings - if he does he will later be accused of fraud.
I think you said he is due at college tomorrow but you feel he might not go. I would arrange to give him a lift if you can. You need to talk and you need to listen to him - there is no point saving his college place if he doesn't intend to follow through - that's just more stress. He might be better to get a job and earn some money to support himself if he doesn't want to be at home. He might also find it less stressful with no homework or projects.
Your son has issues to overcome and sleeping on his mates floor is not sustainable. Is his mate also into drugs/dealing? If he isn't he won't tolerate your son for long. If he is then it will be hard for your son to change while he lives there.
I think you should talk to your son and tell him you appreciate he found it hard living at home - but perhaps it is also hard sleeping on his mates floor? He needs to choose whether to live back home or rent a room/lodge using his savings (that you hold). You must allow him to use the savings for this purpose as these savings prevent him claiming benefits to pay for a room. If he chooses to rent a room I would set it up for you to pay his rent direct if you can, from his savings, as if you give him the money he won't pay his rent. Don't let him get his own flat - it will become a hangout for all the local dossers. Tell him he can't afford it/his money won't last long enough. Lodging with all bills included is what he needs ideally.
He will need a weekly allowance to live on also. Not monthly else he'll be skint for 25 days of the month.
Lodging somewhere with mature caring people would be ideal - but they probably wouldn't tolerate your son's cannabis use. Shared houses that would not care about cannabis use will again probably not help your son's lifestyle.
He needs to think hard about what he wants his future to look like- I think all you can do is lay out his choices and the pros and cons of each (and hope it never occurs to him that taking his 30k, or whatever, and blowing it in 6 months is an option.
Please try to help your son choose his path taking into account his health needs and strengths and weaknesses. The old saying - if you measure a fish by it's ability to climb a tree it is always going to fail. What life choices will actually work for your son?
Wishing you all the best.