I can offer some critique, but first; you are doing great.
The nose and the eyes need to be brought down, to closely match the aggression in the concept art, the goal being you need to introduce more aggression, currently you are actually creating something that is closer to a friendly thing, I am talking a very slight adjustment here.
This is of course if you are intending to mimic the source concept art as much as possible.
Also the back area should be brought back or extended a bit to match the concept art if this is your intent, and the lips should be slightly more linear which would more firmly address the aggression that the original concept art had portrayed.
If you continue the way that you are, this guys will look great so do not be discouraged in any way
I worked on your feedback. That did help increasing the aggression, made the lips linear,too. but very slightly as it messes up texture. but that works!
