Even so ... parents' house, parents' rules.
If [person] seeks other rules, seek other location.
I am totally down with "house rules" like no smoking in the house or within so many feet of the house, etc., but what this adult does with their own time is their business.
People rebel when you put pressure on them.
The best thing his parents can do is stand a bit back, put down some basic ground rules and wait for when he falls, if he falls.
If he doesn't learn from his mistakes, he is never going to learn.
If he succeeds, then so be it...
If people don't try, then they become curmudgeons at a young age and that is a terrible place to be.
There is plenty of time in this kids future for him to lose hope in humanity, society, the world, but right now he's 19, invincible, and full of hope. Let him try some things out and get an idea of how things work before he's chained to a cubicle for the rest of his life.
We're so focused on protecting people, that we forget, sometimes you have to fall in order to learn some of life's lessons.