That's a bad spot to have pain, this is your acromioclavicular joint, where your clavicle, (collarbone), meets the acromion process from your scapula, (shoulder blade). If this joint comes apart it is a shoulder separation, and it is a very difficult injury to recover from. Be very careful with this injury, don't do anything to make it worse, or you'll be out of action for a long time.
You will have to do most of your upper body exercises with very light weights for several months. Don't do any exercises that cause any pain.
A really good chiropractor can make certain that the joint is in it's proper alignment.
I've suffered through this many years ago, and it can be a very long process. But with light weights and higher reps, you will keep most of your muscle mass.
Work the muscles of your rotator cuff group thoroughly and carefully, this will help.
Hopefully if you take care of the problem right now, your recovery can come soon.
If he does have an AC Joint separation/sprain he'd need to see a doctor to make sure the joint is still intact. If it is and its just a sprain, then he needs to stop with the weights, stop using it for about 2-3 weeks and then regain movement and fluidity in the shoulder. after that he should restart with the weights but aiming for perfect form and less weight. That is, if its a Type 1-2.
Anything worse, where the joint is physically separated would probably take surgery. Luckily he doesnt feel pain downward from the clavicle which means its nothing more than a type II.
have you fallen on your shoulder recently? I dont know if you watch any football, but two prominent Quarterbacks have been injured from the same thing, albeit worse, from simply falling on their shoulder. Its a common injury and any type of doctor should be able to help you out. A chiropractor or a physical therapist should be able to show you some shoulder exercises to help heal it.