I know exactly one person (among a dozen or so) who has successfully day traded.
He was a strange character. A social hermit, obsessive and maths/beating gambling systems was very much his special interest. He was a maths genius and the kind of person who wrote code as a hobby to automate winning at online poker competitions so day trading was just another challenge to him. He kept his day job too.
Every other person lost money, some quite a lot. Two of them have unhealthy gambling habits still, one ended up divorced because of it. These are all university educated, otherwise 'intelligent' people.
I would have a serious talk with him. Tell him he needs to continue the a levels so he doesn't leave with nothing. Why not complete just the two maths a levels. Drop physics rather then fail it. That will leave him plenty of free time for day trading on a dummy account.
I don't know how far you are willing to go but I may tell him you won't support him out of education. You don't have to house him beyond 18 etc unless he completes the a levels. If he continues regardless despite the ultimatum then you could withdraw support. No food cooked for him, no laundry, no lifts, no pocket money. All the nice things he would get as a supported child are removed. You need to cut back anyway. Maybe this is too harsh I don't know.
Another option. He needs to prove his plan is workable. (Which may fall through) He needs to show you the graph that indicates a consistent success rate on a dummy account. 2/3 months of growth, not the amount of money he 'earns' but a percentage of growth that shows it isn't just luck but skill.
I'm hoping this very pie in the sky for him and all it takes is one big failure for him to realise the world doesn't work like he wants it to.