Well, in the current position of his hand, I don't think his thumb would show at all. Or he should be purposly bending it forward so strangly. I tried it, it's not impossible, but uncomfterable, and not a position I would use when fighting. I suggest erasing the thumb, since to me it seems the most logic position for it would be behind his hand/other fingers. You should try the position once, so you can see for youreself. Furthermore this is looking sweet, keep it up.