Ug, I hated grades 5 - 8. Going to school was miserable. At that age, all anyone wants to do is fit in, and the easiest way to feel like you fit in is to do what everyone else is doing.
You're really taking this way too seriously. Your son comes to you and says he'd like to try some different underwear. Rather than saying, "Sure honey, here's $20. Go pick out some underwear you like," you are trying to rationalize it. It's not a big deal. He wants to get some new underwear. I'm glad my mom never told me I couldn't buy a certain type of underwear. I know what style feels comfortable on me, so that's the style I buy.
There's a good chance that no one would actually pick on him. They may not even notice. That does change the fact that it's on his mind. He's noticed what all the other guys are wearing, and he wants to fit in. That's all.
In the next few years there will be other ways he wants to "fit in." I think it's important not to make a big deal about the ones that really aren't a big deal. Underwear isn't a big deal.