First of all, can I say that I appreciate your post? I'm a 49 year old man who suffered with gynecomastia my whole life until I finally realized I didn't have to. Here are my before and afters (afters were only 4 days after surgery; I'm now a year after and look totally normal):
http://www.makemeheal.com/pictures/viewAlbum.php?albumid=3307 Both before and after my surgery, I have been wearing UnderArmour compression shirts and vests. They look good, feel good, and they have a certain cachet about them since athletes like them. Depending on how pronounced your son's gyne is, they could be a real benefit. They're available at many large sports stores like Sports Authority, male thingy's Sporting Goods, etc. and they run true to size (you don't need to buy one size smaller, in other words).
Another option is wearing swimming shirts specifically designed for the pool. In that case, be sure also to buy the correct size. I could barely squeeze into one when it was dry, so I bought one two sizes too big for me. As soon as I got in the water it became ridiculously baggy - I had to just give it to Goodwill.
I was mortified as a teenager - it's such a tough time - with any situation where I had to go shirtless. It got easier as I got older and got chest hair. Do your son a HUGE favor and get a consultation with a VERY GOOD, EXPERIENCED plastic surgeon - i.e., one who has experience specifically in gynecomastia surgery, not just surgery in general (and definitely not one who's primarily a women's breast surgeon and wants to do some men as a sideline!). It's critical to get the gland removed: that's what causes the male breast to appear puffy. Before surgery, my breasts were always either puffy or hard depending on how cold it was outside. Now they're always smaller and hard like most men's chests are.
One more thing: as you know, true love doesn't care about appearances. As you can see, I had very pronounced breasts before the surgery but I didn't have any problems with having wonderful relationships with women who cared about me. But I never mentioned my problem to anyone until finally, after 7 years of marriage, I screwed up the courage to ask my wife whether we could afford to have the surgery. She said I didn't need it as far as she was concerned, but if it meant a lot to me, yes, go ahead. I love her so dearly for her acceptance - both of me as a person before and after surgery, and her acceptance of my wish to have the surgery.
Best wishes to you and your son! Feel free to send me a private message if you have follow-up questions. I really feel for your son and what he's going through.