With DS's CTF, I found it hard to resign myself to the fact that it's his money to spend as he wanted, but I'm really glad I didn't object and just let him spend it (I definitely wouldn't have made the choices he did, but it wasn't my money and I had to keep repeating that to myself as a mantra!).
It sounds like you've both got enough to be dealing with, OP, and it sounds like DS is quite reactionary with his behaviour and pushing against you.
The CTF money doesn't have to be another battleground and ultimately, he can spent it how he likes, regardless of what you say anyway.
You know he'll do the opposite of whatever you advise anyway, so I'd just stay out of it completely and wait for him to receive the letter. You don't even have to tell him it's coming and he doesn't have to necessarily associate it with you at all.
Of course, you might want to remind him that his inheritance from x relative makes up the bulk of the money. But I think that the less he sees it as something you're giving him with expectations attached, the less he'll feel the need to weaponise the situation to wind you up.
Having been in similar situations with strong-willed teens, IME the more you try to guide them in the right direction, the less successful it tends to be.
I think that If he thinks you're not fussed how he spends the money, he's less likely to lash out and spend the lot on stuff that he knows will upset you.
I'd look at it this way:
If you do make a fuss about the CTF money, there's a 99% chance he'll spend it on weed, as that would be something he likes that he knows you absolutely hate.
But if you don't say anything about the money and he just gets a letter from the bank one day, there's a higher chance he'll see it as his and nothing to do with you, and therefore his options of what he could spend it on increase massively. He wouldn't see it as anything to do with his intense clashes with you any more- it's just some money.
Sure, he might still spend it on weed, but at least you'll have saved yourself a whole heap of energy and upset, and you'll have one less contentious issue between the two of you.