Hi Karstnator, you probably have already noticed that there are some proportion problems in the drawing: one leg is much longer that the other, his lower torso disappears into the legs (he doesn't seem to have a waist), his right knee is too much inside the leg, the torso at a certain points flattens out weirdly and it seems like he doesnt have a pec.
The most evident issue though is the perspective, for example the hand in the front has the same size of the hand in the back. Same thing with his right arm, it seems flat and too short probably because the upper arm's muscles aren't properly placed. I know that it's very hard to obtain a good 3d effect with poses like this so you shouldn't feel bad if the first times it doesn't come out right.
The face on the other hand is good, you got that mostly right and it's solid overall.
Did you use a reference?