S.T.
It could very easily be a fatty cyst or a regular cyst - do not stress out about cancer - that is VERY VERY unlikely. If you're uncomfortable about it go to another doctor or an endocrinologist. It's probably not something serious - but even cosmetically it should be looked at. Summer is coming and he's going to want to be at the beach or in the pool and the boys will make him miserable if he's got a "breast" or anything close. Good luck!