Most Helpful Guy
Just be honest and tell him. Tell him that you don't feel like it is going anywhere, that you have been thinking about it for a while and that you think you should separate. Don't water it down, just be honest. Don't offer friendship if you aren't committed to that and/or you think it will be too complicated.
It may seem more cruel to be clinical about it but it's actually the kindest thing to do. If you do it in half measures then you leave hope which can prolong the impact. Get it done and make yourself very clear.
Most Helpful Girl
If it's long-distance and you won't be able to see him in person any time soon then a phone call is the next best thing. He should at least be able to hear you say it.