Can a cop charge you with something after the fact?

The cops pulled my boyfriend over and his friend had a half of bud, which they found and charged him for. Then the cops told my boyfriend that if he took them to our house and gave them all his weed paraphernalia they wouldn't charge him. He did give it to them and they left but asked him to call them back in 10 days. He called and they said that they will charge him with what they have previously caught him with if he doesn't give them information (basically to snitch), which he isn't going to do. Is this true, can cops charge you for something after the fact? When they gave no citation,ect.? This isn't the first time this happened either. The first time they found a quarter of bud in my car, but still let us go because they assumed it wasn't ours and that it was the felon in our car who had it. Since then they've been following us and it eventually led up to the situation I originally described. What to do??????? He doesn't want to snitch but doesn't want charges either! Are the cops just trying to scare him or can they back up what they're saying? I live in North Carolina if that helps any. I'm just scared, and I hate being followed. I need to know our rights!!!! Thanks! Additional Details I understand that they can arrest you anytime after they've charged you. That's obvious. Im asking if they can CHARGE you for something they caught you with after they have already let you off with it with no citation,ect. If they really wanted to arrest him i'm sure they would have done so already. And I really don't need stupid answers like "just stop smoking". If I could use a time machine and go back I would, but i can't. What's done is done. I don't care about the future, we have already quit. I want to know what is going to happen NOW with the situation at hand.