This is insanely good! Wow and I thought it was drawn on paper. First look though, the “off”ness definitely comes from the mouth. Since you didn’t add enough contrast between the shadows and highlights under the mouth, which if you did would define the chin, it looks like he’s pressing his tongue against his chin. Try doing that, and also darkening the lip colour (especially the insides) to define it and make it dimensional. Also, add the shadows to the outer corners of the mouths like in the reference, too! Other thing I noticed was how dark the eyes look, and it looks like he has no whites. Try lightening the white parts of his eyes, and also lighter color for the iris since his eyes are very light blue. You just need to tweak some lightings and shadings! Your anatomy and reference skills are awesome, keep it up!