Nice theme for a cartoonish head. I really enjoy it, but here's what I think that you can add/change:
- The mouth area is unshaded, add slight wrinkles under it/next to it;
- To go with the flow, add some scars on his face;
- Add some detail on the grey hair, it will look awesome after you do.
Well, that's my opinion. I'll rate it a 7.