Hi Jill
Sounds like one of those annoying swines we all get sometimes.
I have a couple of questions:
First, what dose he do for a living, and secondly, dose he work out?
You mentioned that his posture was bad, in what way?
If the pain intensifies when he lifts the shoulder past 90 degrees then it is probably supraspinatus.
Check the insertions of pectoralis major and minor and coracobrachialis at the front of the shoulder.
Try this, first lie him on his front and massage the upper back muscles espacialy trap' for a few mins. Then lie him on his back and pinch trapezius, then feel for the top of the scapula and palpate along it with your thumb pushing trapezius out of the way until you find the knot(around lev scap' probably), then gently apply pressure until it is as much as he can comfortably stand then hold it for 20 sec's then slowly release and massage, repeat this process at all tender points from the top of the arm to the base of the neck.
It is more likely one of the muscles under trapezius that is the problem, and not trap' itself.
If (as in most people) his chest muscles are stronger than his upper back muscles then there is a 'tug of war' that the upper back muscles lose, causing them to become tight and painful, its only a few fibers, but they never get to rest.
But definitely check the muscles at the front of the shoulder.
Also levator scap' is worth checking and serratus anterior.
Don't forget that there are a lot of muscles that affect the shoulder, and they all affect eachother, even the ones at the front. pain is often felt in the weakest link in the chain, but that doesn't mean that the problem originated there, it may be that hes had it for years but only now is feeling it at the weakest point. Can you move his shoulder through the full ROM while he relaxes without him feeling any pain?
At the end of the day, probably the best thing the man can do is strengthen his upper back muscles.
If he has had it for a long time then it will take you a while to fix it, but it will only be temporary, you need to identify the root cause (imbalance most likely between the opposing muscles) and advise him how to fix it permanently.
Don't doubt yourself, people should take better care of themselves.
Well I hope Ive been of some help at least.
Kind regards
Shaun