Brachial Plexus pain after flu vaccine

My 74 yr old father received a flu shot on Thursday. The next afternoon, within a 2 hour period, while watching TV, a new pain started in his left shoulder became severe enough that he went to the ER. Dad is diabetic, had neck surgery 20+ yrs ago to remove some bone spurs from his cervical spine (no problems since), had a heart attack 17 yrs ago, and quit smoking the day of his heart attack.
His main concern at the ER was a second heart attack. The ER said it was not his heart, but was likely a muscle spasm. After the weekend, he called his family doctor who refused to see him and said he should continue to take his muscle relaxer and Vicodin - neither of which helped with the pain. On Wednesday, he went to see his chiropractor who said he need to get urgent attention from an orthopedic doctor because a nerve was completely cut off. Symptoms are numbness in his ring and pinkie on his left hand. Shoulder pain between his neck and shoulder, but down his back a few inches and also inside his left scapula. He also has no strength in his left hand and of more concern, if he raises his left arm above his head, then tries to bend it at the elbow toward his head, his arm falls to his head - no ability to use his triceps to hold his bent arm up.

The only med that helps with the pain are ibuprofen.

An MRI of the cervical spine revealed stenosis, but no obvious bulging disc. Doc said that the stenosis didn't seem new. The orthopedic surgeon ordered a second MRI on Monday to view the bronchial plexus. Today he is getting an EMG of his left arm to help identify the nerves involved.

My main concern is cancer in his left lung - pancoast. How likely is that?
Also, what other conditions would cause these symptoms? could this be related at all to the flu shot the day before onset?

Follow-up with the back surgeon is scheduled for tomorrow at 9 am to review the 2nd MRI and the EMG results.

thanks... very nervous