16 yo aren't grateful, it is the nature of the beast, but in your case I see his point. I wouldn't be grateful to people who seemed bent on screwing with my education.
You're on his back "every day" about doing menial work.
You don't mention being on his back about studying. I've tried to find a charitable view of what you're saying. Failed. Please help me.
He's finding 6th form hard. In a way I find that encouraging, since it may well mean he's working hard.
Screwing with that by making him do menial work to add to his load, strikes me as just bizarre.
As it happens I'm dead against work for A level students based upon my own experience.
The sort of work a 16 yo can get is typically working in a supermarket, which is not the goal we have for our kids, and certainly a world away from any experience that will be useful to them.
Also it sends the wrong message big time.
His disposable income will go up dramatically, and typically his social life will improve a great deal with it, and with older people. That means alcohol, and staying out late, and does enable him to afford drugs, and the fags you don't like.
Also supermarkets often put pressure on kids to do weekday work because it's hard for them to fill.
It will be different from boring school work and thus attractive.
Thus you are in effect showing a kid an "easy" way to make good money that's fun, but that impacts his studies and has no viable career path.
The only experience I want my kids to have of the "work environment" is how crap most of them are.
As for throwing darts in the door, well he'll take that skill and rent himself out to AlQaeda as a silent assasin. If that's the worst he's done, you are well ahead of most parents. But you're his parent are you ?
So sending him to his mum will be for the best, since you will then be able to focus upon your children, and not an ungrateful cuckoo.
Maybe your SS will end up getting an adult job serving food to my kids ? That's what you will achieve, but of course it wil be his fault, so that's OK.