I agree that backing off the subject FOR NOW is a good idea, as long as he knows there are options out there if it doesn't go away on its own. On the other side of this, though, is the possibility that it doesn't go away. I'm 40 and went through high school and college with gynecomastia. It's not the same as you get older with gyne. In high school and college, you run into situations where taking off your shirt is expected, such as when a PE teacher has a "shirts vs. skins" game. I prayed that I would be on the shirts team. In high school and college male competition for girls is in high gear, and don't think for a minute gynecomastia isn't going to be used by other males as something to point out as inferior in your son. This is done by bringng it up and laughing about in front of girls and your son. This will be dealt with more if your son goes to college because males will be living together and it's harder to hide.
As for myself, now that I have a lovely wife and two fantastic children (ages 9 and 6) I hear myself telling my kids that I don't feel like getting in the pool with them. They beg me to get in the pool and play, but I'll only do it if there aren't any other people around. Believe me when I tell you that's worse than all the ribbing I got in college put together.
I've recently talked to my mother about this and she did ask my doctor about it when I was about 15. The doctor told her it was best to leave it alone and not to talk about it. Actually that may have been best at the time because they didn't have the ability to do such a clean job of it like they can do now. The point is, in hindsight, I would have preferred that my mother could have talked to me and the doctors once in a while to see if things had changed either in options or in my life regarding my gynecomastia situation. I now have an appointment for next week to speak to a surgeon, but I'm sad that I went through so many years of gyne that have affected the people I love most. I can't get those years back. Please keep the communication open with your son. Keep the communication between your son and his dad CLOSED. Comments like that coming from a father are not normal, and makes me think there could be some sexual abuse potential there. Be VERY careful with that situation. Don't take any chances.