Really awesome piece! I looked at the reference you supplied and took the painting into Photoshop. I was able to do a loose paint over with a little more definition in his face to help you along. I had a hard time with the mouth around the cigarette so I'll let you the artist work some of that business out.
His head is definitely square but I felt that the face was a little too pointy. I just broadened the nose that I referenced and kept his brow heavy, eyes squinting. I hope this helps. Everything else is dynamite.