how old is your boyfriend? I don't know why that matters, but it might put it in context a little more. Anyway, realisitically, what do you actually think you can do to stop him? If you broke his promise to him, and told someone, firstly, who would you tell, and secondly, could they make him stop? If his friendship means a lot to you (and i think it does) then you have to be sure that telling someone would make him stop, otherwise, you've broken your promise, he's angry with you, but still a dealer.
Does he have any aspirations beyond where he is now? does he want to go to uni, or go travelling? maybe if you remind him that there is something beyond what he is doing now, it will help him to realise this is only temporary.
Does you boyfriend use coke much? to quote Scarface 'rule number 2: don't get high off your own supply.' Anyway, what I mean is, if he's both dealling and using, this may become a massive problem. If he only deals, then it may be easier for you to stop him.
tbh, I think the only way he is going to stop if he realises for himself that it's too dangerous and not worth the risks: I think you just might have to help him do that.