Good...I have that on one side as well. It is very easy to dislocate the collar bone at the chest and once dislocated, it stays that way. The docs told me that there is no way to really put it back in so it will stay. I never even felt the dislocation and found out about it when my doc asked how I did it. I have no idea.
The collarbone is the chest side of the AC joint that's on top of the shoulder and connects with the shoulder blade in your back. It helps support the shoulder muscles. So moving your arm upwards really makes it pop out even more. Looks like a knob on your chest below the chin...doesn't it.
If he was born with one dislocated and the other a partial, then you probably never even noticed it until now. Or he could have done it recently but it is almost painless. And until pointed out, you might never notice it. I have only one side so I have a bigger bump that's higher than the other side. The only problem with it...makes my necklaces seem off balance.
Anyhow, there is no fix and it doesn't cause any problems....just a weird thing. Now your son can freak out other kids by showing them his neat bumps in his chest.