decide where your light source is
draw a circle where you want it.. if its coming above, below, side, angled..
the visual will help you remember and understand your lightsource better
determine what parts overhang over others, or stick out. what parts are furthest from the lightsource?
your lightsource seems to be coming from the front(slightly infornt of image) top left corner.
since the (our left) arm is behind him it should be darker and getting lighter as it gets to the fingers.
behind the cheek bone and eye shoudl be shadowed because they stick out blocking the light.
dont think of the image as flat lines. you almost need to visualize its depth to see how far things stick out (causing shadow behind) or overhang (such as the jaw, casting a shadow on the neck and upper torso.. but lightening as it goes down).
to create an effect lightsource you cannot colour the lines straight across with using same depth and light quanitites. you decide upon the variables to add form, texture, atmosphere, etc.
right now it seems mostly like you have a light circle in front on his nose, belly, arm, and as it goes out it darkens (since the only way to go is right, right side darkens.) its similar as if you drew a large cirlce with the edges dark and it lightens as it gets to the middle, then you placed the lines over it and made a few touchups.
hmmm.. think of bone structure... yes yes its a cartoon.. but that makes it easier. then you technically dont have anything to be told your wrong, or poor proportions. lets take the mouth for example.
by the horn and nostril, it widens to allow room for 2 notrils, base for the horn and a wide opening for the mouth. then in the middle it would hollow alittle like your cheeks do when you relax your jaw. as it goes back out to the cheekbone the face widens again. the light would shine brightest on the muzzle by the big horn and by the cheekbone (in front). the middle of the muzzle would be slightly darker since it caves in from each of the wider parts. the lower lip would be darker too because the top part catches most of the light.
these are just suggestions and ideas..
lightsource comes from understanding basic shapes, some 3d forms, and a good imagination.
thats about all i can say..
for now.