You already mentioned that you weren't too pleased with these drawings, but I thought I'd point out a few things anyway. n__n
The shoulders seem *very* narrow. If you think about how the skeleton actually works, you have a rib cage, a collarbone on the front side of your body, and your shoulder blade on the back. Your arm comes out of that. It doesn't look like your character has space for a rib cage, with how narrow his shoulders are.
SINCE I FAIL AT EXPLAINING WHAT I MEAN, have a redline.
The hips are also very narrow, but are still the same width as the shoulders. This is probably a stylistic choice, and I think it might work with wider shoulders.
He's really thin, like... Skeleton thin. I'm not sure he has space for all of his guts in there. I think what's throwing me off is that you have a bit of an hourglass shape happening, which is more of a female characteristic than a male characteristic. If it was straighter from his shoulders to his hips, I think it would feel like a stylistic choice and not make me want to give him some food and fatten him up, poor thing.
His stance. Even if you're not going to see the whole figure, I'd suggest at least sketching out how he's standing, so you can see if it looks like he can balance or not. In the first one, on the left, I think his weight is on the leg to the left, and he's balancing with the other. But his feet are so close together, that he would have an incredibly hard time balancing. The redline on the right shows how I imagine his feet would look. If his knee wasn't bent quite so much, it's look a lot more natural, like what I've done with the first on the left drawing.
I've drawn what I'm suggesting, but I'm still not happy with where that leg on the left is. Sigh. But you can kind of see what I mean, I hope. I also gave him some shoulders, and evened out the chest a little, since it looks like the chest on the right side of him is smaller than what's on the left. I also made his head a little bigger, because it looked too small with his shoulders wider.
I also tried to show that his hips were at an angle, with the one supporting his weight higher than the other. I should have given his spine more of an 's' curve, contrapposto style. THE FOLLOWING LINKS CONTAIN NUDE SCULPTURES, SO NSFW. There is a link for it here, but it has lots of nude greek statues. Be warned!
I checked so many other links, and unfortunately the technique really originated in Greece, with nudes. This is a picture of a nude David, but it has some lines drawn on it that show the concept really well.
The arm on the right... Foreshortening is incredibly difficult. If you look at your arm in the mirror, doing the same pose, I think you'll see the bottom of the elbow below the hand. If not, I would have him hold his hand slightly higher or lower, so it's a little easier to draw. Don't make it harder on yourself than it needs to be! The way it is now, it looks like his hand is growing out of the top part of his arm. :S
I've lifted it a bit in the redline, so you can see the elbow.
With his embarrassed pose, I understand that you're trying to close him off, and brought his legs closer together. Using body language like that is a great idea, and really reads as what we do when we're embarrassed! He still looks a little tippy, though. If I continue the lines of the legs, I think he's actually crossed his legs...
The redline shows what I mean. I couldn't tell which leg was forward, since the tones on the pants are so similar, so I just guessed. Is he a ballet dancer? I could believe that a dancer could balance like this.
With the third drawing, with his arms up, I again gave him some shoulders. I left the head the same size as your original drawing, and also tried to keep him as thin, so you could see options.
Hopefully this helps! It was fun to do.