OP no reason to be depressed. This is not the end of the world. I wouldn't tell him about the ADD correlation to scare him, but more to explain why you are worried. If you get beneath your anger, you'll see what's really behind it is worry. If you convey that in a caring way, it may be more effective than harsh punishment. That may just alienate him. I've always knows of a connection btwn impulsivity and addiction (drugs, gambling, etc.--but this doesn't happen to everybody so don't worry!) so I don't have a specific article in mind but I just googled impulsivity addiction and got this list--go through it and find something straightforward and appropriate to share:
https://www.google.com/search?q=impulsivity+addiction&ie=utf-8&oe=utf-8 The UCLA page is interesting. I would also discuss with him what he thinks the consequences of getting caught doing that at school would be. I would worry about the school's response if he got caught smoking pot at school. Pot is not as much a drug of addiction as other drugs, as I understand it, but you could share a concern about pot being a gateway to drugs that are addicting and his potential vulnerability due to impulsivity. I've explained to my impulsive 13 y.o. that he will have to be more careful than the average person if he ever experiments with drugs or drinks alcohol (at any age) because of these risks.