Line art looks pretty solid, although his arm is pretty small, his cheek is really pointy (could chalk it up to style), and the lines on his glasses look sort of weird. I don't like the gradients, though; if I were you, I'd either shade it normally or just use solid colors, because this actually makes him look flatter than solid colors, imo. The gradient implies his face is one plane, whereas a solid color just looks like you didn't shade it. Also, the light's inconsistent; the stuff on his face is coming from the upper right, whereas it's coming from the lower right on his chest.
If I were you, I'd try actually coloring it. (Even if it looks bad, you won't get better unless you try!) You can start with simple cel-shading and just using one crisp shadow color if you want to work up to something more complicated, and it would fit well with the cartoony style.