Thumbs can be hard since to be honest they are longer than we generally think, they just start down closer to the wrist than fingers do. One of the things that can greatly change the length appearance is how your fingers curved and the flap of skin connecting your pointer finger and your thumb. Here's sorta what I'm talking about. (Looking at your hand in the pose you want in a great way to improve this.) Also don't forget details like the wrinkle lines on your knuckles, these can help visualize fingers.

Ditto what monke said about the hat. Imagine it moved down so that the brim is just a little above his eyebrows.
Only other thing I'd work on is the eyes. Everyone draws manga eyes differently, but I'd still give them a more definition, like pupils. Solid color makes them looks glassy.