first off you've done a hell of a job so far. I think his eyes should be more of an olive green color rather than a blue. I think his jowls come down too far. its especially apparent in the first image. I think your diffuse could use some work as well. The red feels a bit too thrown on I would look at different images of his face and how the reds are distributed as well as the overall tone. I hope that helps. Keep at it you've done an awesome job so far. other people can probably give more in depth critiques on the face I don't really do characters/faces much.
I'd love to see more angles,I think Polygons brought up a lot of good points,I'd also like to add that your rendition has sort of a sweet old man look that I dont see on the actual bill murray. Furthermore the surface hair doesn't look to have ddifferent lengths, or thickness (thicker hair for beard area), his eyebrows looks very neat, and I'd also love to see some nosehair/earhair.some positives:I reeally like the specular level, the shading of his hair and the skin detail on his forehead and neck.keep it up!