He is a grown up. You’re not his mother. Yes, he’s also a coward for choosing that way of telling you.

Wait until he's in a work meeting and then text him that you've decided to start shooting heroin.

I think the true FML here is OP's opinion of what constitutes appropriate behavior between partners. If the BF is so afraid of OP's reaction, there is a huge problem that actually needs either counseling or breaking up. In a relationship, you don't get to control the other person's behavior. That's a passive aggressive dick move. Best you can do is decide whether or not you can live with their behavior, and then adapt accordingly.

Prove him wrong!!! Tear him a new one while you're on the bus. During the pandemic, the other riders will probably enjoy the show, seeing how live theater has been closed down.

Go for a run with him to discuss it.

If yelling is your first response to anything then you deserve it

Well maybe if you weren't so predictably explosive he would have told you in person.

I have to admire his committment to killing the whole pack. It's not easy to smoke that much.

I think the bigger issue here is that your SO has a serious addiction, not when he decided to tell you.