I chucked dss out a year ago, when he was 20. His behaviour was nowhere near as bad as this, but the constant rows over him not pulling his weight, and the fact that he had failed to pay his digs for six months were enough for him to go. I worried like hell about him, he has always been really young for his age, and I was convinced he would get eaten alive I the real world, but it has been ok. I gave him £500 for a flat deposit, but he spent that on a new phone and sofa surfed for a while. I just had to let him get on with it. He has his own flat now, and we all get on much better. He phones for advice when he gets into difficulty, but has really matured. It was best for us all in the long run, but the guilt I felt was enormous at the time.
You just need to get it done. Set a date, give him a set amount of money to start him off, and then back off. He will no doubt screw things up a few times and make some bad choices, but you have to let him get on with it or he will never learn how to be an adult. He will probably hate you for it, but tough. If you are lucky, a few years down the line he will realise what a shit he has been.
I think, with some male children in particular, there comes a point where they need to leave, but are scared to do it without a push. Whilst they stay in the home, they engage in a pissing contest to try and become the alpha man, by throwing their weight around. Either you give in and let them run the house, or you throw them out of the pack. Think of it like a pack of chimps or lions. The young male needs to leave for the health of the pack, and needs to set up on their own. If they were sweetness and light, there would be no motivation to get them out. Your Dss clearly sees you as the alpha, since his dad has changed for you, so you are the one whose power he needs to topple.
Get it done, the relief you will feel a few months down the line is unbelievable. It's your only option for a health family dynamic in the future.