Can I tell my PhD supervisor honestly why I want to leave him I spent more than half a year with a PhD supervisor, and am not happy with both aspects: relationship and academically. With time, it becomes obvious that our field of research doesn't match. He reassured me that he had sufficient knowledge on my topic, but it turns out that he doesn't. I'd say it's the fault on both sides (I should've checked his publications more). 

The biggest problem is that he doesn't want to admit it, and either only gives me a very general advice, or blames my draft as not academic enough and tell me to revise the structure before he agrees to read it, and then he'd again give a general advice. He also prefers to meet only once every two or three months. I see no career prospect here, as he'll retire in three years, and then his department will close (I couldn't have known that before).

I consulted another professor that I met in a conference, who also specializes in my topic, and he said that my research is solid (plus, he gave me valuable inputs that I have never received from my own supervisor). Finally, learning about my problems, the second professor offered to supervise me, and said all I need to do is to apply at his university (in another state). By the way, I have neither working nor funding contract with my current university. 

I know it sounds not very ethical, but I said nothing to my supervisor until I'm officially accepted by the new university. He's old, proud, and quirky, often taking the slightest mistake (by anyone) as a conspiracy against him. He never admits he doesn't master my topic, so he'll blow a fuse if I tell him that when I announce about my leaving him. I simply don't know what to answer if he asks me why (and he will certainly do). Any suggestion how to handle this well? Thank you.
My suggestion is that you talk about all of the reasons you are moving to the other university, as opposed to leaving the current one. These reasons could be to do with relationships (e.g., family) or future opportunities (e.g., a postdoc) or being paid whilst you are currently unpaid. If you have to give a reason why you are leaving your current position, the least-harmful choice seems to be to mention that his lab will close and that you chose to study with him in the hope of staying within his wonderful group.

The choice comes down to the problems of being honest (which does not imply you should be dishonest). If you are honest, then because he will retire even if you magically changed his world-view, then the impact would still be negligible. However, you may burn bridges and although a small probability, the professor may take retributive action.