Things i think would improve:
With that big a breath, i think it should move it's head up and back. Take a deep breath and see how your upper body shifts back as your lungs inflate :)
As he inhales, i think the fold below his mouth should disappear, like when a fat dude sticks his belly out he tends to get more rounded and show less folds :)
The hat could have a better defined volume, by better usage of lighting to show the cilindrical shape.
Shadows on the back of the slug should change as the body shape is changed with the lungs inflating.
You have a low contrast shading on the body, but go for complete black on the hat. That steals attention from the body and makes it a bit harder to read. You should attempt to keep the whole palette at the same level, using more contrasting colors only to grab the attention of the viewer to a specific place.
Hope this is helpful!

I do see the outline color change from a green to blue. The stomach should move slower and should come in less.

Will fix soon. Broke my finger.

yeah, for sure the stomach is moving, but try to make him less hard, make him move a little more on the other parts, like his back or head.

Yes, but move less.

I wanted to move the stomach.

First of all, the stomach moves way too much. It should be more subtle and slower. The way you have it now, it looks as if he's ran a long race and is gasping for breath. Also, some of the outline color changes on one of the frames.