Thank you both for your replies.
I think you are right, I really ought to tell him. I think it's all the more daunting in that by telling him, I am finally going to be admitting to myself that I am definitely poly. No more kidding myself that I could do monogamy and live the happily-ever-after fairy tale that I've had ingrained in me.
I think the chances are that he's going to want to break up, but I guess I'll have to wait and see. Maybe he'll surprise me.