If this guy is fantastic, and you really enjoy being with him, I'd say let it go!
Don't let the thong be the elephant in the room - say something, tease him a bit about it, have fun. Don't lose out on having somebody great in your life, because of their underwear and no-show socks. And don't let your 'friends' drive somebody wonderful out of your life, because they feel like they have to make fun.
I think sometimes that we're so caught up in what everybody else is wearing, and it holds us back as an entire gender. Women tear down other women based solely on their clothing and appearance, and then we wonder why men don't take us seriously in the work place, or pay us equal wages, or consider childcare and welfare issues, when they create social policy. We want men to be more sensitive, and understanding and accepting of women. Shouldn't it go both ways? I can't help but think that this is a good place to start -be supportive when your guy friends dress outside of the 'norm'.
Plus, he's got a really great physique, and totally pulls off this look. I give him props
