Looks pretty good! Only a couple things I can comment about. For one, the lower jaw seems to protrude a bit too far, making it look like this fellow has a bit of an overbite. His chin is also a little too thin/flat, so try bringing its outline to match the jaw a little better. Other thing is that the nose leather is a tad too big vertically for how big it is horizontally. Try lengthening it down the face a little more to bring it into proportion.

And one thing that doesn't count really since this is a sketch, try to bring out some more definition in the neck. I'm guessing that this is a fluffy fellow but it would be good to show some shape in the neck anyway, be it where the actual neck is, the tufts of fluff, the shapes within the neck, etc. Other than that, this looks great!

My general advice is to break the dubious into simple geom. shapes.
Next on, give em a look and deduce how much of a shape[surface] is exposed to/hidden from view.
You'll know how to place your proportions then. Takes training, but it's a sure way.