As a sort of disclaimer, I have worked in animation for ten years, seven of those years as a supervisor/manager, so critiquing animation is kind of my thing.
Overall, this looks really nice. You've got some surprisingly effective uses of foreshortening and perspective, particularly as far as bringing the knuckles toward the camera go. The shapes are more or less consistent throughout, and you haven't broken any limbs or done anything terribly jerky. The facial expressions are nice, and the drapery on the shirt is also done very well. Good job all around.
As for the issues with the piece, the main thing I notice is that his hips float around. It's true that they would probably move, but I don't get the sense that they are attached to his legs and moving in a natural way. If a person's hips move side to side while they are standing still, they necessarily tilt inwards toward the centre of gravity, and here I'm seeing a bit more of a slide.
Also when he takes the shirt off, I am left with the distinct impression that the camera has been dropped. The angle switches from pretty much front-on to suddenly looking up at him. This is further emphasized by the line of his shorts and shirt hem suddenly curving upward, and the way his chin and hairline change. Perhaps this is intional, as there are several things contributing to it, but it's somewhat awkward with a character just floating in the aether. The angle switches again as he lowers the shirt after removing it, giving the impression that he is now looming over the viewer.
The second thing I would point out is something called overshoot. You've used it in a few places already, but I think you could use it in a few more. In particular, the movement of hands nearly always requires overshoot, as they are so complex and have so many degrees of separation from our body mass.
You may understand this already but I will explain regardless; overshoot is the idea that when our bodies come to rest, we never get the final position quite right, we usually go too far and then have to make a correction in the opposite direction to end up at the final position.
You can see that you have done this with the arm that lifts the shirt at the beginning, and that's good, but I believe the opposite arm could use some of that treatment as well, particularly as he lowers it after showing off his midriff.
Anyway, if you are not an animator, the fact that you're doing this is quite impressive. I have worked with people who have training in animation whose work was not this nice, and I'd have been quite happy to have had someone with this quality of work on a production team.
Thanks for sharing it!