USF Alum, is that University of San Francisco, the Jesuit School or University of South Florida, sort of a big difference in culture!
Starting when my daughter was around 12 years old she would have some huge drama episode over some minor problem. I would ask my wife what was our daughter’s issue; she would tell me, I would shake my head in disbelief and say “Why that is nothing to be upset about.” My wife’s answer was always the same, “You have never been a 12 year old girl.” There in is the problem of relating to your son’s issue, you have never been a 15 year old boy. No matter how empathetic you are, in his eyes you are never going to fully understand his plight. So you will do the very best you can to try and understand, from his standpoint, what it is like to be a 15 year old boy with a problem he is absolutely certain no one else has or will understand. You will get some of it totally wrong but you should get enough right that he will understand that you care and have his back. Have empathy for him.
Be honest, but not too honest. We all know the person that will give it to you with both barrels, the kind of person no one likes. Brutally honest isn’t typically as fully honest as the person thinks, but it is always brutal. So be totally honest and don’t say things that you don’t believe, but say them with compassion. You can say the glass is half full and still be honest. Your son knows you pretty well by this point in his life; he can most likely tell if you are being insincere.
No teenage boy wants to talk to his parents, especially his mother, about his body. So if you can get him to open up, listen and encourage him to talk to you. But don’t obsess about the subject; nagging him to talk about it will typically cause him to withdraw. He will cope with his problem one way or the other; from what you said hiding it is his preferred method. If you can get him to talk about it, it brings light to the subject and he no longer needs to hide it alone. If he can share this problem with someone, even if that is not you, it will be the first step towards his acceptance and seeing his breast development as an annoyance not something shameful or embarrassing.
Knowledge is power. Knowledge empowers a person; it gives them the feeling of control over their life. You did not say if he even realizes he has gynecomastia. If he doesn’t realize what causes the problem that can be stressful in and of itself. If you can get any amount of dialog going with him, I suggest you give him a link to this site. He can view the stages of gynecomastia, read what causes it—basically too much testosterone all at once, that should be a surprise to him. Maybe even suggest that he posts his story, feelings, and photos. He will get feedback from others who are going through the same thing or have gone through it. Plenty of teenagers his age and younger post; they get a sense of perspective and “hey I’m not the only one.”
Knowing what to say to your children is always tough, especially if it is the opposite sex child. Try what I have suggested, I don’t know your son but I do know what being a 15 year old boy with breasts was like. Let us know how it goes. Parents occasionally post but I don’t know of any that have ever given us a follow-up. I am certain there are other parents that would like to know how your journey goes.