If you need, write a letter and have them read it in your presence or sit them down and tell them directly.
This is like ripping off a bandaid. I know it hurts, but trust me - and I know this from painful experience - breaking up indirectly, just letting the relationship whither down because you don't want to face it is far more painful for the other person and probably for you as well. If you care about them and if you "have a set", you'll tell him directly. (Oh, and if there's no fear of a violent reaction, of course.)
It sucks, but it will help you both grow. Good luck!