I'm here to echo both MAVW and Zia.
You have a habit of working with really static poses and anatomy, everyone's limbs look like sticks and they always stand in that same stiff posture! While as a general thing I'd reccomend you take a look at some anatomical guides on google so you can better work the contours of the shoulders, arms, and legs, here's some ideas on how to spice up the design slightly without compromising that simplicity!
All I did here was add a couple colors for shading and change the shirt into a hoodie, to give him a more grumpy schlubby look. The rest was all slight pushing of anatomy, like giving his pants a slight bell-bottomy look to imply calf muscles, which tend to bulge out a bit below the knee at most angles!
Just reading about some of these in your off time can give you a lot of information, I'm pretty lax on my anatomical studies (more than I probably should be) but just glancing at this stuff in passing gives me a lot to work from.
Try studying how shoulders, hips, and spines work especially, those are probably among the most important for expressive poses!