I need help. I'm young but this is serious.?

I'm 14. I've been dating this guy for a month now. He's in juvy & he gets out in 17 days. A part of me misses him then the other part is just really pissed off at him. He smokes weed & I never knew this until now. I'm disappointed he didn't tell me & he's little sister told me everything he's done.. I thought he was the best boyfriend ever! But I guess I was wrong. Should I break up with him? Please help me with this.