I divide a leg into three parts - thigh, shin and foot, - and for each frame I just rotate every part to the correct position. First I do it for one leg (which is at the front) and then I just copy it for the second (back) leg and make it a bit darker then the front leg. (The same goes for arms).
Also I think your posture doesn't look stable enough, it looks like he is going to fall back, I would move the upper part of the body a bit to the right and tilt it a bit so he wouldn't look so straight, and if you say that he is supposed to be tired, maybe his head should look down?
I took one of the frames and made some rough changes here:

So the main point - don't be afraid to bend his knees more

And don't forget to rotate foots and hands accordingly to the position of the legs and arms. Now his hands remain the same on all frames, try to rotate it