Re: A Dillema...
This is a terrible spot to be in. Obviously, you care about him, yet you do not want to betray his trust in you. You have to decide which is more important to you, and frankly, I do not envy your situation here. No matter what you do, you will likely feel lousy. There is no good compromise, short of talking with him and getting him to agree to stop smoking (which he may or may not do, or be truthful about).
Pot is bad stuff (Our 'Serial Thrilla' is a drug abuser so he's not exactly objective here), it would be best for him to stop. If you discuss your concerns with him and he refuses to take your concerns seriously, i'd suggest you take a long walk alone somewhere and think about the 2 options (saying nothing and living with him continuing to use drugs, or telling YOUR parents not his), and decide which one of those options you can more easily live with.
Alot of times in really close relationships people tell us stuff that upsets us or makes us concerned, esp. about the things that they might be doing that's not good for them. It's important to establish with them (maybe as they're saying it), that you will not keep secrets when it comes to their health and safety. That way, at least they know what the potential consequences of telling you are.
here, however, you're faced with a hard decision. Think about which alternative you can more easily live with.