It is really hard to break up with someone without hurting their feelings. However, it’s better to hurt him now than to continue dating him even though you see no future in your relationship. As always, being honest and direct is the best way to do it. He’ll certainly understand you, if you are right about him being decent and nice. If he doesn’t accept your point and raise a fuss about it, then you are entirely wrong about him, and entirely right to break up with him.
What’s important is that you are straightforward and honest with your feelings. He’ll appreciate that. He might get hurt but he’ll get over it and go on with his life so don’t feel so guilty about it.