Advice needed

 my boyfriend is currently addicted to weed and I know it's his decision and he should be able to do what he wants but I'm worried that, that's all he cares about and he's going to ruin his life, I don't know how to bring it up in conversation without sounding controlling or selfish
I just really need some advice about this, if anyone's been in a similar situation and it worked out for you feel free to comment it :)