If the relationship has been decent, yes you owe him an explanation. You owe it to yourself to give one too.
This is one of those times when you aren’t gonna be sure what to say, which is part of your problem. The tried, trusted and trite “It isn’t you, it’s me” and “Lets stay friends” will want to come spilling out of your mouth at some stage.
Don’t let them.
If you like the guy well enough, but he is simply not the one for you, tell him the truth. Explaining it will hurt. Bowing out without explanation will hurt more.