to be brutally honest... the animation doesn't work at all. you should try to make the head and the arms follow the body, cause at the moment it's just kind of random. i see that you almost got it when he's on the way up, so if you try doing that on the way down as well, and do some small changes it would be nice-oh!
It's a nice image but theres some things i've noticed that could be changed to make it a lot smoother. I hope you don't mind some critique.
The rise and fall of the arms seems a bit out of sync with the body, especailly when his chest moves up, you could try and resync them together but give the arms a more smoother decent so they don't snap as hard. There's no real weight to the cloth or the body parts that are moving, making it look stiff, try adding in some bounce to bring more life to the animation. The legs don't move in tandem with the chests rise and fall which makes it seem his body is just floating over it, try stretching the legs up a bit when his chest rises but keep his feet where they are. Some tiny bends in the knees when he moves up and down could also work to bring a much smoother appereance.