Hopefully you're still considering the modeling stage because I picked up on a few more things.
This isn't a paint over it's more to assist me in pointing out the parts that could be corrected.
1. The mouth area. (blue)
You've made the top area of his mouth slightly concaved. Judging from the ref images it's not really this way.
From the cheeks-- when it hits those blue lines it should actually start to protrude from his face. Overall I think this is what's throwing his mouth off. It's not just the bottom lip that sticks out, it's the entire mouth. The furthest it goes out is underneath the bottom lip, but not as much as in your model. Do you know what I mean? If not let me know and I'll try and clarify.
2. (Green) and (purple)
The green bit goes down a bit too far and is a bit too exaggerated, I suggest making the transition from lip to chin smoother. Also, the (purple) jaw goes down a bit too far. In your model I get the impression his jaw is much bigger than I see in the ref images.
3. nose (red)
I haven't gotten this right at all, but it at least points out the areas which don't seem right. The from plane of the nose seems to widen a bit more in the middle of his nose in the refs.
Also, I'm not sensing that ridge on the tip of his nose, maybe that's just because of the lighting.
4. eyes (yellow) and eyebrows (orange)
Your eyes are slanting downwards too much on the the outer side. His irises are too big (you might fix this later) and the eyebrows should be closer to the eyes.
It's not a very good paint over, so don't take my word for all of these things it's just the areas that don't feel right to me.
Still looking great, I want to see this to the end!