It is quite messy this, but do you really want to dump your boyfriend just because you want his mate, put it this way you cannot have the best of both worlds it does not work like that, surely first thing to do is try and make a go of it with your current boyfriend because usually when you start wanting his mate then things can't be right with your current boyfriend, try working out with him first.
But should you go with his mate, least dump your current boyfriend before making a move on his mate, either way though your boyfriend will be very hurt, does he really deserve this?
I mean first what is it that your not happy with your boyfriend he may not know at all, so perhaps have a talk with him, it works wonders