This could be anything from a cyst to a muscle knot to a tumor mass to a bone out growth.
It could also turn out to be a harmless skin condition.
To make an accurate diagnosis a doctor will have to examinr it and investigations done if required.
If he has had no other associated symptoms of fatigue/weight loss/pain etc. there may not be cause for concern.
Do keep us posted on the outcome.Regards and God bless.
did you find out what it is, bc i have the same thing??
37 yr old has mas on left side behind ribs, pain and some weight loss