What Causes Round Painful Bruise On Shoulder?

My dad does not take good care of himself, bad teeth, stomach problems, H-Pylori and who knows what else. He was recently working hard chainsawing wood about three days ago now and was complaining of a sore arm. Today he noticed on his shoulder at the top of his arm, a round bruise with another color in the middle. He wasn't hit with anything but it's very painful. He's one of those people who would not complain unless it was very bad...but he'll put off going to a doctor.