trying to keep this anime boy pose simple but it looks stiff

TOJI FUSHIGURO!!! (that's his name btw)

Hey there! You could improve the gesture of the pose a bit, since keeping the body completely upright makes it feel a little stiff and takes away some of the dynamism. Try tilting the shoulders slightly, and as a result, the torso as well. In a pose like this, the body naturally leans to one side to maintain balance (or at least that's what happened when I tried to mimic the pose, lol). You could also raise the leg a bit higher and tilt the hips slightly in the opposite direction of the shoulders. Doing this will make the pose feel much more natural and less rigid. Hope this helps! :)

gesture might not be the main thing here, proportions look okay to me. it reads stiff mostly because both arms are doing the same shape and the shoulders stay level. maybe drop one shoulder and tilt the hips the other way, and let the raised knee cross a bit past center