His dealer calls because he knows it works. Even if that guy weren't calling, there are a hundred more to replace him. Delete his number? It's memorized. Change the phone number? That won't last long.
Does your boyfriend even want to quit? Before you tie yourself to him and babysit, ask yourself if he even wants to quit. You can't make him quit. You said he's selling his subs. He told you he doesn't want to drag you down. Doesn't sound like he's going to NA meetings or doing treatment. Doesn't sound to me that he doesn't want to stop.
Do you want to be his babysitter? He's a grown man. I thought I could babysit my husband too but he still managed to get high. Finally I just started living my own life.
I wish you well.