I know it's an esoteric topic for many.
And good questions!
Beyond sleeves, men's kimono are shorter in length. Men don't blouse their kimono over a hidden cord to make a fold across the abdomen ("ohashori") like women do.
For the kimono itself, these are the only guaranteed ways to tell if one belongs to a man: bottom outside sleeve corner shape (just about a 90 degree angle for men), closed backs on the sleeves, and shorter length. They wear their family crests ("kamon") in the same places as women, and fold left over right like women. Colors are not a guarantee of men's or women's because women can wear the same dark colors as men, and once in a while you'll see a light, pastel men's outfit.
Here's a Tumblr post I did you might find helpful: it features various looks for a man's formal outfit, which includes the haori coat and hakama pants (these are worn over the obi, not under).
Men's obi: Yes, they wear a much thinner obi than women of any age. Their obi are called "kaku obi", and are most commonly tied into the "kai no kuchi" knot, which is worn in the back just like women's knots. They also wear them lower than women and at a bit of an angle, higher in the back than the front. Here's a side-by-side drawing of men and women's looks:
Regarding your character concept, I would say the most obvious thing to show could be the triangle shaped "hat" tied to the front of the head. It's not always used, but that's a classic sign a character in a play/drawing/story etc. is a ghost come back from the dead. Also, remember to make his kimono collar sit right over left (like a backwards "y" when you look at it). Left over right is for the living (both men and women) and right over left for the dead. Men and women wear the same kimono in death, as a note.
One more note is that a normal obi isn't worn: it's more of a loose, thin sash (tied in the front/off to the side rather than the back like normal obi, which makes sense if you remember that another person is dressing the corpse).
Here's a Japanese classical "ghost" (yuurei 幽霊) party costume for reference.