Origami, Tanya is giving you really good advice here.
I don't know about you, but I always tried to make everything equal with my two sons. They could poke me every single time by saying I did something for one that I wasn't doing for the other. I reacted. It worked. I am a big "fairness" person. So, both of them knew very well what to do and say to get what they wanted. Over these past years, as you know, both of my sons took very different paths. Different circumstances call for different reactions from parents. We can't act the same when the situations are different, and as we learn more, we can do better. Most of us know on this forum that our doing for our grown adult children isn't good for them or for us. Stopping is still very very hard, even though in our intellectual minds, we can see clearly that this isn't good.
Your son knows how to push your buttons, and he's going to do it every time. I finally learned to say this: I'm not going to debate it with you. I've decided and I'm not going to change my mind. I'm sorry if you don't think my decision is fair.
Don't even go there with him, because you'll just go round and round and feel worse and worse. Say your piece and walk away. Our grown kids don't have to like or agree with what we say or do, and believe me, you're not going to cause anything to be worse than it already is. We just don't have that kind of power over them.
I will never forget taking my son to rehab in a city four hours away. I dreaded the ride, and I dreaded leaving him there, and I was very emotional. After I took him there, he asked the people at the rehab place if there was a place to buy cigarettes there. They said no. He then turned to me and said, Mom will you go and buy two cartons of cigarettes and bring them back here? I just have to have something.
And guess what? I did it! I drove to a convenience store, bought two cartons of cigarettes to the tune of $90 and took them back. After I did it, and while I was driving home, I regretted that action. I swore at that point I would never again buy him cigarettes and I haven't. I know it is the lesser of the evils, that it's a smaller battle, etc., etc., etc. I see all of that. But i'll be darned if I'm going to ever again be the agent of him smoking.
If you must buy him anything, buy only things that will actually and truly help him, like food (you don't want him to be hungry), transportation, like a bus pass or a bicycle, a pair of shoes to go on a job interview or something that actually will ADD to his success, not detract from it. I know it's hard to tell them no about these kinds of things, but we have to slow down and get really really clear about what we are willing to do and what we aren't willing to do...without manipulation and interference from them. And then, even harder, we have to stick to it.
You are dealing well with all of this. It is really really hard to set boundaries when we are exhausted, scared, frustrated and sad about our precious adult DCs. But again, it is the best thing for them and for us when we can. Go slow, and take it one day at a time. It doesn't have to happen overnight.