It is his top, either his jacket (which would probably not get tucked in) or his shirt. Which depending on the country should or should not be tucked.
For example, in the US military the BDU tops (shirts) are not tucked in, but they do not hang down like that either.
I found several other images in a few secs which included some who looked the same way.
My guess is this, but it is just a guess.
That it is his jacket. And, they they can opt to wear the jacket or not. And if they opt to wear the jacket it hangs down some as in the images.