Is there any real harm in him trying them? If he’s openly discussed it with you, & is actively seeking your input, then I think that speaks volumes for not only his confidence, but your relationship as a whole!
What’s the worst that could happen? He doesn’t really like them in reality, or perhaps you don’t? No big deal.
Who knows, you might discover you like seeing his bum cheeks exposed, or perhaps he does? If that’s the case, it will only lead to good things!
Short answer - Yes. Many men from all walks are wearing thongs, they are becoming far more popular now, & there are all sorts of wonderfully different styles available! If I could offer any suggestion: Let him choose 1, you choose 1 & decide on a 3rd that appeals to you both, for whatever reasons.
For context, I’m a straight & very happily married man. When I initially even thought about trying a thong or similar, I was far more anxious & nervous about the idea than was ever warranted! I had all sorts of unfounded & misguided fears about taboos & outdated misconceptions, & felt inclined to keep it all to myself. I struggled with the idea for ages! When I finally decided to act on my thoughts, I tried them on in private, still fearful & questioning everything… Once I saw how everything looked, I was actually pleasantly surprised! I finally mustered up the courage to wear them on a date 1 night with my GF (now Wife), & her reaction later that evening was nothing short of amazing! She giggled with sheer happiness & surprise, & wouldn’t actually let me take them off for quite some time… She wanted to inspect every angle, the look, feel & fit of them!
Fast forward to now, I always seek her opinion when shopping for the next potential “sexy little number” & I think she feels wonderful that her opinion counts? She doesn’t always get what she wants, sometimes it’s purely about me, but in almost every scenario they are often removed before long, so everybody wins! I say go for it! Life is far too short not to try! Even if you’re not 100% sure about the whole idea, support his bravery & encourage his willingness to try! Just my honest thoughts…