I know what you mean. I have one like that too.
As far as self-medicating with weed... it DOES work for anxiety - that much, even the medical community agrees on too. But even RX medications don't solve the problem - they just buy time to learn new skills. Anxiety doesn't just go away, we have to change how we think so we can change our reactions. And weed kills any motivation to learn the new skills.
Weed also calms the impulsivity of ADHD - again, not the most effective drug, but it does work. Problem is, it's not targeted, it's more general, so it has other side effects (like killing motivation). It's hard to get specific dosages using weed so some days you have more than you need and some days less. And for most people, there are really effective ADHD medications out there.
So, if he "needs" weed to function, then what he really needs is RX medications and a good therapist.
Not that he's likely to agree with me on that.